Ink set for inkjet recording and inkjet recording method

ABSTRACT

The ink set for inkjet recording and an inkjet recording method in the present invention, which can provide a recorded image having high quality and ensure that the obtained image can maintain the image quality with less change in the color balance even when stored in an oxidative atmosphere, and which includes at least three kinds of inkjet inks, each of which includes a coloring agent dissolved or dispersed in an aqueous or oily medium and has a maximum absorption spectrum in a spectral absorption region different from each other, wherein, when a photographic printing is performed on a reflection-type image-receiving medium using the ink set and a forced discoloration rate constant with an ozone gas of each ink is determined in each printed region of said at least three kinds of inks, the ratio of any two of the forced discoloration rate constants is from 0.5 to 2.0.

FIELD OF THE INVENTION

The present invention relates to an ink set for inkjet use and an inkjet recording method, which can provide a recorded image having an image quality less susceptible to the storage environment and ensure an excellent storage stability of image.

BACKGROUND OF THE INVENTION

Accompanying recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper, film, cloth or the like not only in offices but also at homes.

The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure using a piezo element, a system of jetting out a liquid droplet by generating bubbles in an ink under heat, a system of using an ultrasonic wave, and a system of jetting out a liquid droplet by suction using an electrostatic force. The ink used for such inkjet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks, an aqueous ink is predominating in view of production, handleability, odor, safety and the like.

The coloring agent used in such an ink for inkjet recording is required to have high solubility in a solvent, enable high-density recording, provide good color, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing on an image-receiving material and less bleeding, give an ink having excellent storability, have high purity and no toxicity, and be available at a low cost. However, it is very difficult to find out a coloring agent satisfying these requirements in a high level. Various dyes and pigments for inkjet use have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found at present. Conventionally well-known dyes and pigments having a color index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording and a coloring agent for inkjet use having excellent color hue and satisfied in the fastness has not yet been obtained.

As the color image-forming agent having excellent durability, a pigment is generally superior to a dye, however, in the case of a dispersion-type ink using a pigment, the image is inferior in the transparency (particularly, in the high-density area) or image quality. Therefore, neither a dye nor a pigment can satisfy both the fastness and the high image quality at the same time. In recent years, a problem arises that the image formed by the inkjet system is readily discolored particularly by ozone present in the environmental atmosphere and even a phthalocyanine dye known to be a fast dye is discolored. This discoloration reaction greatly affects the color balance of the image. In particular, when a reflection image is formed, discoloration due to ozone proceeds in aging during storage and since the proceeding of discoloration differs every each coloring agent constituting the image, this gives rise to a problem that with the progress of discoloration (reduction in the color density), the image loses the color balance.

Such deterioration in the image quality of the recorded image with the passage of time is accelerated, for example, by high temperature, high humidity, illumination in high brightness or exposure to an oxidative atmosphere such as ozone gas. Therefore, although it is of course important that the ink in the ink composition is excellent in all the heat stability, light fastness and oxidation resistance, the above-described disruption of color balance cannot be overcome merely by rendering a coloring agent for a specific ink to be fast.

SUMMARY OF THE INVENTION

The present invention has been made under these circumstances and an object of the present invention is to provide an ink set for inkjet use and an inkjet recording method, which can provide a recorded image having high quality and ensure that the obtained image can maintain the image quality with less change in the color balance even when stored in an oxidative atmosphere.

In order to attain the above-described object, the present inventors have made extensive investigations on the relationship between the fastness of coloring agent and the change of color balance in an oxidative atmosphere with regard to a reflection image printed using respective inks of an ink set. As a result, a coloring agent requirement for reducing the change of color balance has been found. The present invention has been accomplished based on this finding. More specifically, the present invention provides an ink set for inkjet use and an inkjet recording method described below.

1. An ink set for inkjet recording, comprising at least three kinds of inkjet inks, each of which includes a coloring agent dissolved or dispersed in an aqueous or oily medium and has a maximum absorption spectrum in a spectral absorption region different from each other,

wherein, when a photographic printing is performed on a reflection-type image-receiving medium using the ink set and a forced discoloration rate constant with an ozone gas of each ink is determined in each printed region of said at least three kinds of inks, the ratio of any two of the forced discoloration rate constants is from 0.5 to 2.0.

2. The ink set for inkjet recording as described in the item 1, which comprises at least one cyan ink, at least one magenta ink and at least one yellow ink.

3. The ink set for inkjet recording as described in the item 1 or 2, which comprises at least one black ink.

4. The ink set for inkjet recording as described in any one of the items 1 to 3, which comprises two cyan inks, two magenta inks and two yellow inks.

5. The ink set for inkjet recording as described in any one of the items 1 to 4, wherein the coloring agent is a dye.

6. The ink set for inkjet recording as described in any one of the items 1 to 4, wherein the coloring agent is a pigment.

7. The ink set for inkjet recording as described in any one of the items 1 to 4, wherein the coloring agent includes a dye and a pigment.

8. The ink set for inkjet recording as described in any one of the items 1 to 7, wherein the ratio of any two of the forced discoloration rate constants is from 0.7 to 1.4.

9. The ink set for inkjet recording as described in any one of the items 1 to 7, wherein the ratio of any two of the forced discoloration rate constants is from 0.8 to 1.25.

10. The ink set for inkjet recording as described in any one of the items 1 to 9, wherein each of the at least three kinds of inkjet inks contains the coloring agent in an amount of 0.2 to 20 wt %.

11. The ink set for inkjet recording as described in any one of the items 1 to 10, wherein the at least three kinds of inkjet inks contains a compound represented by the following formula (1):

wherein A represents a 5-membered heterocyclic group;

B¹ and B² each represents a nitrogen atom, ═CR¹— or —CR²═, and when either one of B¹ and B² represents a nitrogen atom, the other represents ═CR¹— or —CR²═; R⁵ and R⁶ each represents a hydrogen atom, an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an alkylsulfonyl group, an arylsulfonyl group or a sulfamoyl group, and the hydrogen atom of each substituent may be substituted;

G, R¹ and R² each independently represents a hydrogen atom, a halogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, a carboxyl group, a carbamoyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a heterocyclic oxycarbonyl group, an acyl group, a hydroxy group, an alkoxy group, an aryloxy group, a heterocyclic oxy group, a silyloxy group, an acyloxy group, a carbamoyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, an amino group, an acylamino group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a heterocyclic sulfonylamino group, a nitro group, an alkylthio group, an arylthio group, a heterocyclic thio group, an alkylsulfonyl group, an arylsulfonyl group, a heterocyclic sulfonyl group, an alkylsulfinyl group, an arylsulfinyl group, a heterocyclic sulfinyl group, a sulfamoyl group or a sulfo group, and the hydrogen atom of each substituent may be substituted;

R¹ and R⁵, or R⁵ and R⁶ may combine to form a 5- or 6-membered ring.

12. The ink set for inkjet recording as described in any one of the items 1 to 11, wherein the at least three kinds of inkjet inks contains a compound represented by the following formula (I):

wherein X₁, X₂, X₃ and X₄ each represents —SO-Z, —SO₂-Z, —SO₂NR₁R₂, a sulfo group, —CONR₁R₂ or —CO₂R₁; Z represents an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group or a heterocyclic group; R₁ and R₂ each represents a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group or a heterocyclic group; when a plural number of Zs are present, the plurality of Zs may be the same or different; Y₁, Y₂, Y₃ and Y₄ each represents a monovalent substituent; when a plural number of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s are present, the plurality of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s may be the same or different; M represents a hydrogen atom, a metal atom, or an oxide, hydroxide or halide thereof; a₁ to a₄ and b₁ to b₄ each represents the number of substituent X₁, X₂, X₃, X₄, Y₁, Y₂, Y₃ or Y₄, and a₁ to a₄ each represents an integer of 0 to 4 but all of a₁ to a₄ are not 0 at the same time; b₁ to b₄ each represents an integer of 0 to 4.

13. An inkjet recording method, comprising forming an image using the ink set for inkjet recording described in any one of the items 1 to 12.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in detail below.

The forced discoloration rate constant with an ozone gas, which shows the oxidation-resistant fastness of each ink in the ink set of the present invention, is described below.

The forced discoloration rate constant of each ink as used in the present invention is determined as follows. An image is obtained by photographic printing on a reflection-type image-receiving medium using only a pertinent ink in the ink set, and the color region having a color of the main spectral absorption region of the ink and having a reflection density of 0.90 to 1.10 as measured through a filter of Status A is selected as the initial density point. This initial density is designated as the start density (=100%). Then, the image is discolored using an ozone discoloration tester where the ozone concentration is always kept at 5 mg/liter. The time until the density becomes 80% of the initial density is measured, and assuming that the relationship between the discoloration density and the time is in accordance with the primary reaction rate formula, the discoloration reaction rate constant is determined from the time measured. Accordingly, the discoloration rate constant determined is a discoloration rate constant in the color region printed by the ink, but in the present invention, this value is used as the discoloration rate constant of ink.

The reflection image printed for the purpose of measurement is a step patch (so-called color patch) where the density of each single color of the ink set is stepwise varied, and a color patch having a density corresponding to the initial density (1.0±0.1) in the printed color patch is selected for the measurement portion. The reflection density is a density determined with measurement light transmitted through a Status A filter by a densitometer satisfying the international standard ISO5-4 (geometric condition of reflection density).

In the measurement of the forced discoloration rate constant with ozone gas, an ozone generator capable of stationarily keeping the ozone gas concentration at 5 mg/liter in the test chamber (for example, a high-pressure discharge system of applying an a.c. voltage to a dry air) is used and the aeration temperature is adjusted to 25° C. The forced discoloration rate constant with ozone gas is an index for the easiness of oxidation due to oxidative atmosphere in the environment, such as photochemical smog, automobile exhaust, organic vapor from coated surface of furniture or carpet and gas generated in architrave of a bright room, and this index uses an ozone gas as a representative of these oxidative atmospheres.

The forced discoloration rate constant of each ink, which is obtained as such, is a characteristic value determined by the ink and the reflection-type image-receiving medium. However, when the reflection-type image-receiving medium used is an image-receiving paper, film or cloth for inkjet recording, the forced discoloration rate constant is almost determined only by the ink and less affected by the image-receiving medium. Accordingly, in the present invention, unless the kind of the image-receiving medium is particularly important, the value of forced discoloration rate of ink is shown without describing the image-receiving medium.

The ink set of the present invention and each ink constituting the ink set are described below. The ink set for inkjet use of the present invention comprises at least three or more inks. Each ink contains a coloring component. The ink set preferably comprises at least one cyan color, at least one magenta color and at least one yellow color and also preferably contains a black ink. The ink set more preferably comprises two cyan color inks, two magenta color inks and two yellow color inks.

In an image recorded using such an ink set, the discoloration rate differs according to the density, color tone and contrast in respective regions of the image and this causes distortion in the color balance and makes it difficult to maintain the excellent image quality obtained immediately after the image formation.

Particularly, in the present invention of forming an image on a reflection-type image-receiving medium, the Lambert-Beer's law for the transmissive material is not established. The dissociation between the amount of discolored dyestuff and the decrease of density extremely increases particularly in the low-density region. In other words, a region where the amount of existing dyestuff and the density are not in a linear relationship is widely present.

In addition, when two or more inks are used for each color constituting the ink set, the discoloration rate differs between respective inks having the same or analogous color hue but differing in the density and this brings about conspicuous change in the color tone.

Therefore, in the case where the image is exposed to an oxidative atmosphere represented by ozone gas, deterioration of image and shortening of image life are caused not only by the reduction in the density accompanying the oxidative destruction of dyestuff but also or rather more greatly by the distortion in the color balance.

The present invention is characterized in that when each ink is determined on the forced discoloration rate constant with ozone gas in printed regions of at least three kinds of inks constituting the ink set and differing in the color hue, the ratio of any two rate constants is in the rage from 0.5 to 2.0. In the thus-designed ink set, the color balance of the image is less disrupted even when stored under the condition of oxidative atmosphere such as ozone gas, and the overall quality as the image is not easily deteriorated.

More specifically, in the ink for inkjet recording and the inkjet recording method of the present invention, when each color ink constituting the ink set is determined on the forced discoloration rate constant, no matter what two rate constants are selected at random, the ratio thereof is from 0.5 to 2.0, preferably from 0.7 to 1.4, more preferably from 0.8 to 1.25.

The ink composition for inkjet recording of the present invention contains a coloring agent preferably in an amount of 0.2 to 20 wt %, more preferably from 0.5 to 15 wt %. In the case where two different kinds of inks having the same color hue but differing in the dye concentration, for example, light cyan ink and cyan ink, are used in combination in the ink set, the dye concentration in the ink of lower concentration is suitably from 1/20 to 1/2 by mass, preferably from 1/10 to 3/7 by mass, of the dye concentration in the ink of higher concentration.

The ink for use in the ink set of the present invention is an ink obtained by dissolving or dispersing a dye or a pigment in water or an organic solvent. In particular, an aqueous solution-type ink using a water-soluble dye is preferred.

In each ink for use in the ink set of the present invention, one dyestuff or a plurality of dyestuffs are used according to the purpose and function of each constituent ink so as to adjust the color tone of a full color image. Examples of the dyestuff which can be used include the followings.

Incidentally, the “dyestuff” is used in the present invention when both a dye and a pigment are included and particularly when it is not necessary to distinguish whether the coloring agent is a dye or a pigment.

Examples of the yellow dye include aryl- or heteryl-azo dyes having a phenol, a naphthol, an aniline, a pyrazolone, a pyridone or an open chain-type active methylene compound as the coupling component; azomethine dyes having an open chain-type active methylene compound as the coupling component; methine dyes such as benzylidene dye and monomethine oxonol dye; and quinone-base dyes such as naphthoquinone dye and anthraquinone dye. Other examples of the dye species include quinophthalone dye, nitro-nitroso dye, acridine dye and acridinone dye. These dyes may be one which provides a yellow color for the first time when a part of the chromophore is dissociated. In this case, the counter cation may be an inorganic cation such as alkali metal and ammonium, an organic cation such as pyridinium and quaternary ammonium salt, or a polymer cation having such a cation as a partial structure.

Examples of the magenta dye include aryl- or heteryl-azo dyes having a phenol, a naphthol or an aniline as the coupling component; azomethine dyes having a pyrazolone or a pyrazolotriazole as the coupling component; methine dyes such as arylidene dye, styryl dye, merocyanine dye and oxonol dye; carbonium dyes such as diphenylmethane dye, triphenylmethane dye and xanthene dye; quinone-base dyes such as naphthoquinone, anthraquinone and anthrapyridone; and condensed polycyclic dyes such as dioxazine dye. These dyes may be one which provides a magenta color for the first time when a part of the chromophore is dissociated. In this case, the counter cation may be an inorganic cation such as alkali metal and ammonium, an organic cation such as pyridinium and quaternary ammonium salt, or a polymer cation having such a cation as a partial structure.

Examples of the cyan dye include azomethine dyes such as indoaniline dye and indophenol dye; polymethine dyes such as cyanine dye, oxonol dye and merocyanine dye; carbonium dyes such as diphenylmethane dye, triphenylmethane dye and xanthene dye; phthalocyanine dyes; anthraquinone dyes; aryl- or heteryl-azo dyes having a phenol, a naphthol or an aniline as the coupling component; and indigo•thioindigo dyes. These dyes may be one which provides a cyan color for the first time when a part of the chromophore is dissociated. In this case, the counter cation may be an inorganic cation such as alkali metal and ammonium, an organic cation such as pyridinium and quaternary ammonium salt, or a polymer cation having such a cation as a partial structure.

A black dye such as polyazo dye may also be used.

Examples of the water-soluble dye include a direct dye, an acid dye, a food color, a basic dye and a reactive dye. Preferred examples thereof include C.I. Direct Red 2, 4, 9, 23, 26, 31, 39, 62, 63, 72, 75, 76, 79, 80, 81, 83, 84, 89, 92, 95, 111, 173, 184, 207, 211, 212, 214, 218, 21, 223, 224, 225, 226, 227, 232, 233, 240, 241, 242, 243 and 247; C.I. Direct Violet 7, 9, 47, 48, 51, 66, 90, 93, 94, 95, 98, 100 and 101; C.I. Direct Yellow 8, 9, 11, 12, 27, 28, 29, 33, 35, 39, 41, 44, 50, 53, 58, 59, 68, 86, 87, 93, 95, 96, 98, 100, 106, 108, 109, 110, 130, 132, 142, 144, 161 and 163; C.I. Direct Blue 1, 10, 15, 22, 25, 55, 67, 68, 71, 76, 77, 78, 80, 84, 86, 87, 90, 98, 106, 108, 109, 151, 156, 158, 159, 160, 168, 189, 192, 193, 194, 199, 200, 201, 202, 203, 207, 211, 213, 214, 218, 225, 229, 236, 237, 244, 248, 249, 251, 252, 264, 270, 280, 288, 289 and 291; C.I. Direct Black 9, 17, 19, 22, 32, 51, 56, 62, 69, 77, 80, 91, 94, 97, 108, 112, 113, 114, 117, 118, 121, 122, 125, 132, 146, 154, 166, 168, 173 and 199; C.I. Acid Red 35, 42, 52, 57, 62, 80, 82, 111, 114, 118, 119, 127, 128, 131, 143, 151, 154, 158, 249, 254, 257, 261, 263, 266, 289, 299, 301, 305, 336, 337, 361, 396 and 397; C.I. Acid Violet 5, 34, 43, 47, 48, 90, 103 and 126; C.I. Acid Yellow 17, 19, 23, 25, 39, 40, 42, 44, 49, 50, 61, 64, 76, 79, 110, 127, 135, 143, 151, 159, 169, 174, 190, 195, 196, 197, 199, 218, 219, 222 and 227; C.I. Acid Blue 9, 25, 40, 41, 62, 72, 76, 78, 80, 82, 92, 106, 112, 113, 120, 127:1, 129, 138, 143, 175, 181, 205, 207, 220, 221, 230, 232, 247, 258, 260, 264, 271, 277, 278, 279, 280, 288, 290 and 326; C.I. Acid Black 7, 24, 29, 48, 52:1 and 172; C.I. Reactive Red 3, 13, 17, 19, 21, 22, 23, 24, 29, 35, 37, 40, 41, 43, 45, 49 and 55; C.I. Reactive Violet 1, 3, 4, 5, 6, 7, 8, 9, 16, 17, 22, 23, 24, 26, 27, 33 and 34; C.I. Reactive Yellow 2, 3, 13, 14, 15, 17, 18, 23, 24, 25, 26, 27, 29, 35, 37, 41 and 42; C.I. Reactive Blue 2, 3, 5, 8, 10, 13, 14, 15, 17, 18, 19, 21, 25, 26, 27, 28, 29 and 38; C.I. Reactive Black 4, 5, 8, 14, 21, 23, 26, 31, 32 and 34; C.I. Basic Red 12, 13, 14, 15, 18, 22, 23, 24, 25, 27, 29, 35, 36, 38, 39, 45 and 46; C.I. Basic Violet 1, 2, 3, 7, 10, 15, 16, 20, 21, 25, 27, 28, 35, 37, 39, 40 and 48; C.I. Basic Yellow 1, 2, 4, 11, 13, 14, 15, 19, 21, 23, 24, 25, 28, 29, 32, 36, 39 and 40; C.I. Basic Blue 1, 3, 5, 7, 9, 22, 26, 41, 45, 46, 47, 54, 57, 60, 62, 65, 66, 69 and 71; and C.I. Basic Black 8.

In the ink set of the present invention, a pigment may be also used, and commercially available pigments and known pigments described in various publications can be used. The publication includes Color Index, compiled by The Society of Dyers and Colourists, Kaitei Shin Han Ganryo Binran (Revised New Handbook of Pigments), compiled by Nippon Ganryo Gijutsu Kyokai (1989), Saishin Ganryo Oyo Gijutsu (Newest Pigment Application Technology), CMC Shuppan (1986), Insatsu Ink Gijutsu (Printing Ink Technique), CMC Shuppan (1984), and W. Herbst and K. Hunger, Industrial Organic Pigments, VCH Verlagsgesellschaft (1993). Specific examples of the organic pigment include azo pigments (e.g., azo lake pigment, insoluble azo pigment, condensed azo pigment, chelate azo pigment), polycyclic pigments (e.g., phthalocyanine-base pigment, anthraquinone-base pigment, perylene-base or perynone-base pigment, indigo-base pigment, quinacridone-base pigment, dioxazine-base pigment, isoindolinone-base pigment, quinophthalone-base pigment, diketopyrrolopyrrole-base pigment), dyeing lake pigments (lake pigments of acidic or basic dye) and azine pigments. Specific examples of the inorganic pigment include yellow pigments such as C.I. Pigment Yellow 34, 37, 42 and 53, red-base pigments such as C.I. Pigment Red 101 and 108, blue-base pigments such as C.I. Pigment Blue 27, 29 and 17:1, black-base pigments such as C.I. Pigment Black 7 and magnetite, and white-base pigments such as C.I. Pigment White 4, 6, 18 and 21.

The pigment having a preferred color tone for the formation of an image includes the followings. As the blue to cyan pigment, phthalocyanine pigments, anthraquinone-type indanthrone pigments (for example, C.I. Pigment Blue 60) and dyeing lake pigment-type triarylcarbonium pigments are preferred, and phthalocyanine pigments are most preferred (preferred examples thereof include copper phthalocyanine such as C.I. Pigment Blue 15:1, 15:2, 15:3, 15:4 and 15:6, monochloro or low chlorinated copper phthalocyanine, aluminum phthalocyanine such as pigments described in European Patent 860475, nonmetallic phthalocyanine such as C.I. Pigment Blue 16, and phthalocyanine with the center metal being Zn, Ni or Ti, and among these, C.I. Pigment Blue 15:3 and 15:4 and aluminum phthalocyanine are more preferred).

As the red to violet pigment, azo pigments (preferred examples thereof include C.I. Pigment Red 3, 5, 11, 22, 38, 48:1, 48:2, 48:3, 48:4, 49:1, 52:1, 53:1, 57:1, 63:2, 144, 146 and 184, and among these, C.I. Pigment Red 57:1, 146 and 184 are more preferred), quinacridone-base pigments (preferred examples thereof include C.I. Pigment Red 122, 192, 202, 207 and 209 and C.I. Pigment Violet 19 and 42, and among these, C.I. Pigment Red 122 is more preferred), dyeing lake pigment-type triarylcarbonium pigments (preferred examples thereof include xanthene-base C.I. Pigment Red 81:1 and C.I. Pigment Violet 1, 2, 3, 27 and 39), dioxazine-base pigments (for example, C.I. Pigment Violet 23 and 37), diketopyrrolopyrrole-base pigments (for example, C.I. Pigment Red 254), perylene pigments (for example, C.I. Pigment Violet 29), anthraquinone-base pigments (for example, C.I. Pigment Violet 5:1, 31 and 33) and thioindigo-base pigments (for example, C.I. Pigment Red 38 and 88) are preferred.

As the yellow pigment, azo pigments (preferred examples thereof include monoazo pigment-type C.I. Pigment Yellow 1, 3, 74 and 98, disazo pigment-type C.I. Pigment Yellow 12, 13, 14, 16, 17 and 83, synthetic azo-type C.I. Pigment 93, 94, 95, 128 and 155, and benzimidazolone-type C.I. Pigment Yellow 120, 151, 154, 156 and 180, and among these, those not using a benzidine-base compound as a raw material are more preferred), isoindoline-isoindolinone-base pigments (preferred examples thereof include C.I. Pigment Yellow 109, 110, 137 and 139,), quinophthalone pigments (preferred examples thereof include C.I. Pigment Yellow 138) and flavanthrone pigments (for example, C.I. Pigment Yellow 24) are preferred.

As the black pigment, inorganic pigments (preferred examples thereof include carbon black and magnetite) and aniline black are preferred.

Other than these, an orange pigment (for example, C.I. Pigment Orange 13 and 16) and a green pigment (for example, C.I. Pigment Green 7) may be used.

The pigment which can be used in the ink set of the present invention may be the above-described pigment which is not subjected to a particular treatment or subjected to a surface treatment. For the surface treatment, a method of coating the surface with resin or wax, a method of attaching a surfactant, and a method of binding a reactive substance (for example, a radical generated from a silane coupling agent, an epoxy compound, polyisocyanate or a diazonium salt) to the pigment surface may be used and these are described in the following publications and patents:

(1) Kinzoku Sekken no Seishitsu to Oyo (Properties and Applications of Metal Soap), Saiwai Shobo;

(2) Insatsu Ink Insatsu (Printing Ink Printing), CMC Shuppan (1984);

(3) Saishin Ganryo Oyo Gijutsu (Newest Pigment Application Technology), CMC Shuppan (1986);

(4) U.S. Pat. Nos. 5,554,739 and 5,571,311; and

(5) JP-A-9-151342 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-10-140065, JP-A-10-292143 and JP-A-11-166145.

Particularly, self-dispersible pigments prepared by allowing a diazonium salt to act on carbon black described in U.S. Patents of (4) and capsulated pigments prepared by the method described in Japanese Patent Publications of (5) are effective, because dispersion stability can be obtained without using an excess dispersant in the ink.

In the present invention, the pigment may be further dispersed by using a dispersant. Various known dispersants can be used according to the pigment used, for example, a surfactant-type low molecular dispersant or a polymer-type dispersant can be used. Examples of the dispersant include those described in JP-A-3-69949 and European Patent 549486. In using the dispersant, a pigment derivative called synergist may also be added so as to accelerate the adsorption of dispersant to the pigment.

The particle size of the pigment is, after dispersion, preferably from 0.01 to 10 μm, more preferably from 0.05 to 1 μm.

As for the method of dispersing the pigment, known dispersion techniques used for the production of ink or toner can be used. Examples of the dispersing machine include vertical or horizontal agitator mill, attritor, colloid mill, ball mill, three-roll mill, bar mill, super-mill, impeller, disperser, KD mill, dynatron and pressure kneader. These are described in detail in Saishin Ganryo Oyo Gijutsu (Newest Pigment Application Technology), CMC Shuppan (1986).

Examples of the dye which can be preferably used in a magenta or light magenta ink or a dark yellow ink of the ink set for inkjet use of the present invention and is effective in maintaining the image fastness and color balance include an azo dye having an aromatic nitrogen-containing 6-membered heterocyclic ring as the coupling component, represented by the following formula (1):

wherein A represents a 5-membered heterocyclic group;

B¹ and B² each represents ═CR¹— or —CR²═ or either one of B¹ and B² represents a nitrogen atom and the other represents ═CR¹— or —CR²═; R⁵ and R⁶ each independently represents a hydrogen atom or a substituent, the substituent is an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an alkylsulfonyl group, an arylsulfonyl group or a sulfamoyl group, and the hydrogen atom of each substituent may be substituted;

G, R¹ and R² each independently represents a hydrogen atom or a substituent, the substituent is a halogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, a carboxyl group, a carbamoyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a heterocyclic oxycarbonyl group, an acyl group, a hydroxy group, an alkoxy group, an aryloxy group, a heterocyclic oxy group, a silyloxy group, an acyloxy group, a carbamoyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, an amino group, an acylamino group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a heterocyclic sulfonylamino group, a nitro group, an alkylthio group, an arylthio group, a heterocyclic thio group, an alkylsulfonyl group, an arylsulfonyl group, a heterocyclic sulfonyl group, an alkylsulfinyl group, an arylsulfinyl group, a heterocyclic sulfinyl group, a sulfamoyl group or a sulfo group, and the hydrogen atom of each substituent may be substituted; and

R¹ and R⁵, or R⁵ and R⁶ may combine to form a 5- or 6-membered ring.

The dye of formula (1) is described in more detail.

In formula (1), A represents a 5-membered hetero-cyclic group. Examples of the heteroatom of the heterocyclic ring include N, O and S. A is preferably a nitrogen-containing 5-membered heterocyclic ring and the heterocyclic ring may be condensed with an aliphatic ring, an aromatic ring or another heterocyclic ring. Preferred examples of the heterocyclic ring represented by A include a pyrazole ring, an imidazole ring, a thiazole ring, an isothiazole ring, a thiadiazole ring, a benzothiazole ring, a benzoxazole ring and a benzisothiazole ring. Each heterocyclic group may further have a substituent. Among those, preferred are a pyrazole ring, an imidazole ring, an isothiazole ring, a thiadiazole ring and a benzothiazole ring represented by the following formulae (a) to (f):

In formulae (a) to (f), R⁷ to R²⁰ each represents the same substituent as G, R¹ and R² in formula (1).

Among formulae (a) to (f), preferred are a pyrazole ring and an isothiazole ring represented by formulae (a) and (b), and most preferred is a pyrazole ring represented by formula (a).

In formula (1), B¹ and B² each represents ═CR¹— or —CR² ═ or either one of B¹ and B² represents a nitrogen atom and the other represents ═CR¹— or —CR²═. B¹ and B² each preferably represents ═CR¹— or —CR²═.

R⁵ and R⁶ each independently represents a hydrogen atom or a substituent, the substituent is an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an alkylsulfonyl group, an arylsulfonyl group or a sulfamoyl group, and the hydrogen atom of each substituent may be substituted.

R⁵ and R⁶ each is preferably a hydrogen atom, an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkylsulfonyl group or an arylsulfonyl group, more preferably a hydrogen atom, an aromatic group, a heterocyclic group, an acyl group, an alkylsulfonyl group or an arylsulfonyl group, and most preferably a hydrogen atom, an aryl group or a heterocyclic group. The hydrogen atom of each substituent may be substituted. However, R⁵ and R⁶ are not a hydrogen atom at the same time.

G, R¹ and R² each independently represents a hydrogen atom or a substituent, the substituent is a halogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, a carboxyl group, a carbamoyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a heterocyclic oxycarbonyl group, an acyl group, a hydroxy group, an alkoxy group, an aryloxy group, a heterocyclic oxy group, a silyloxy group, an acyloxy group, a carbamoyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, an amino group, an acylamino group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a heterocyclic sulfonylamino group, a nitro group, an alkylthio group, an arylthio group, a heterocyclic thio group, an alkylsulfonyl group, an arylsulfonyl group, a heterocyclic sulfonyl group, an alkylsulfinyl group, an arylsulfinyl group, a heterocyclic sulfinyl group, a sulfamoyl group or a sulfo group, and the hydrogen atom of each substituent may be substituted.

G is preferably a hydrogen atom, a halogen atom, an aliphatic group, an aromatic group, a hydroxy group, an alkoxy group, an aryloxy group, an acyloxy group, a heterocyclic oxy group, an amino group, an acylamino group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an alkylthio group, an arylthio group or a heterocyclic thio group, more preferably a hydrogen atom, a halogen atom, an alkyl group, a hydroxy group, an alkoxy group, an aryloxy group, an acyloxy group, an amino group or an acylamino group, and most preferably a hydrogen atom, an amino group (preferably an anilino group) or an acylamino group. The hydrogen atom of each substituent may be substituted.

R¹ and R² each is preferably a hydrogen atom, an alkyl group, a halogen atom, an alkoxycarbonyl group, a carboxyl group, a carbamoyl group, a hydroxy group, an alkoxy group or a cyano group. The hydrogen atom of each substituent may be substituted.

R¹ and R⁵, or R⁵ and R⁶ may combine to form a 5- or 6-membered ring.

When A has a substituent or when the substituent R¹, R², R⁵, R⁶ or G further has a substituent, examples of the substituent include the substituents set forth above for G, R¹ and R².

In the case where the dye of formula (1) is a water-soluble dye, the dye preferably has further an ionic hydrophilic group as a substituent on any position of A, R¹, R², R⁵, R⁶ and G. Examples of the ionic hydrophilic group as the substituent include a sulfo group, a carboxyl group, a phosphono group and a quaternary ammonium group. Among these ionic hydrophilic groups, preferred are a carboxyl group, a phosphono group and a sulfo group, more preferred are a carboxyl group and a sulfo group. The carboxyl group, the phosphono group and the sulfo group each may be in a salt state and examples of the counter ion for forming the salt include ammonium ion, alkali metal ion (e.g., lithium ion, sodium ion, potassium ion) and organic cation (e.g., tetramethylammonium ion, tetramethylguanidium ion, tetramethylphosphonium).

The substituents used in the present invention are described in detail below. The meaning of each substituent is common among different symbols in formula (1) and also in formula (1a) shown later.

The halogen atom includes a fluorine atom, a chlorine atom and a bromine atom.

The aliphatic group means an alkyl group, a substituted alkyl group, an alkenyl group, a substituted alkenyl group, an alkynyl group, a substituted alkynyl group, an aralkyl group and a substituted aralkyl group. The “substituted” used for a “substituted alkyl group” and the like in the present invention means that the hydrogen atom present in an “alkyl group” and the like is substituted by a substituent described above for G, R¹ and R².

The aliphatic group may be branched or may form a ring. The number of carbon atoms in the aliphatic group is preferably from 1 to 20, more preferably from 1 to 16. The aryl moiety in the aralkyl group and the substituted aralkyl group is preferably a phenyl group or a naphthyl group, more preferably a phenyl group. Examples of the aliphatic group include a methyl group, an ethyl group, a butyl group, an isopropyl group, a tert-butyl group, a hydroxyethyl group, a methoxyethyl group, a cyanoethyl group, a trifluoromethyl group, a 3-sulfopropyl group, a 4-sulfobutyl group, a cyclohexyl group, a benzyl group, a 2-phenethyl group, a vinyl group and an allyl group.

The aromatic group means an aryl group and a substituted aryl group. The aryl group is preferably a phenyl group or a naphthyl group, more preferably a phenyl group. The number of carbon atoms in the aromatic group is preferably from 6 to 20, more preferably from 6 to 16.

Examples of the aromatic group include a phenyl group, a p-tolyl group, a p-methoxyphenyl group, an o-chlorophenyl group and an m-(3-sulfopropylamino)phenyl group.

The heterocyclic group includes a substituted heterocyclic group. In the heterocyclic group, the heterocyclic ring may be condensed with an aliphatic ring, an aromatic ring or another heterocyclic ring. The heterocyclic group is preferably a 5- or 6-membered heterocyclic group. Examples of the substituent include an aliphatic group, a halogen atom, an alkylsulfonyl group, an arylsulfonyl group, an acyl group, an acylamino group, a sulfamoyl group, a carbamoyl group and an ionic hydrophilic group. Examples of the heterocyclic group include a 2-pyridyl group, a 2-thienyl group, a 2-thiazolyl group, a 2-benzothiazolyl group, a 2-benzoxazolyl group and a 2-furyl group.

The carbamoyl group includes a substituted carbamoyl group. Examples of the substituent includes an alkyl group. Examples of the carbamoyl group include a methylcarbamoyl group and a dimethylcarbamoyl group.

The alkoxycarbonyl group includes a substituted alkoxycarbonyl group. The alkoxycarbonyl group is preferably an alkoxycarbonyl group having from 2 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the alkoxycarbonyl group include a methoxycarbonyl group and an ethoxycarbonyl group.

The aryloxycarbonyl group includes a substituted aryloxycarbonyl group. The aryloxycarbonyl group is preferably an aryloxycarbonyl group having from 7 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the aryloxycarbonyl group include a phenoxycarbonyl group.

The heterocyclic oxycarbonyl group includes a substituted heterocyclic oxycarbonyl group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic oxycarbonyl group is preferably a heterocyclic oxycarbonyl group having from 2 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the heterocyclic oxycarbonyl group include a 2-pyridyloxycarbonyl group.

The acyl group includes a substituted acyl group. The acyl group is preferably an acyl group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the acyl group include an acetyl group and a benzoyl group.

The alkoxy group includes a substituted alkoxy group. The alkoxy group is preferably an alkoxy group having from 1 to 20 carbon atoms. Examples of the substituent include an alkoxy group, a hydroxyl group and an ionic hydrophilic group. Examples of the alkoxy group include a methoxy group, an ethoxy group, an isopropoxy group, a methoxyethoxy group, a hydroxyethoxy group and a 3-carboxypropoxy group.

The aryloxy group includes a substituted aryloxy group. The aryloxy group is preferably an aryloxy group having from 6 to 20 carbon atoms. Examples of the substituent include an alkoxy group and an ionic hydrophilic group. Examples of the aryloxy group include a phenoxy group, a p-methoxyphenoxy group and an o-methoxyphenoxy group.

The heterocyclic oxy group includes a substituted heterocyclic oxy group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic oxy group is preferably a heterocyclic oxy group having from 2 to 20 carbon atoms. Examples of the substituent include an alkyl group, an alkoxy group and an ionic hydrophilic group. Examples of the heterocyclic oxy group include a 3-pyridyloxy group and a 3-thienyloxy group.

The silyloxy group is preferably a silyloxy group substituted by an aliphatic or aromatic group having from 1 to 20 carbon atoms. Examples of the silyloxy group include trimethylsilyloxy and diphenylmethylsilyloxy.

The acyloxy group includes a substituted acyloxy group. The acyloxy group is preferably an acyloxy group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the acyloxy group include an acetoxy group and a benzoyloxy group.

The carbamoyloxy group includes a substituted carbamoyloxy group. Examples of the substituent include an alkyl group. Examples of the carbamoyloxy group include an N-methylcarbamoyloxy group.

The alkoxycarbonyloxy group includes a substituted alkoxycarbonyloxy group. The alkoxycarbonyloxy group is preferably an alkoxycarbonyloxy group having from 2 to 20 carbon atoms. Examples of the alkoxycarbonyloxy group include a methoxycarbonyloxy group and an isopropoxycarbonyloxy group.

The aryloxycarbonyloxy group includes a substituted aryloxycarbonyloxy group. The aryloxycarbonyloxy group is preferably an aryloxycarbonyloxy group having from 7 to 20 carbon atoms. Examples of the aryloxycarbonyloxy group include a phenoxycarbonyloxy group.

The amino group includes a substituted amino group. Examples of the substituent include an alkyl group, an aryl group and a heterocyclic group, and the alkyl group, the aryl group and the heterocyclic group each may further have a substituent. The alkylamino group includes a substituted alkylamino group. The alkylamino group is preferably an alkylamino group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the alkylamino group include a methylamino group and a diethylamino group.

The arylamino group includes a substituted arylamino group. The arylamino group is preferably an arylamino group having from 6 to 20 carbon atoms. Examples of the substituent include a halogen atom and an ionic hydrophilic group. Examples of the arylamino group include a phenylamino group and a 2-chlorophenylamino group.

The heterocyclic amino group includes a substituted heterocyclic amino group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic amino group is preferably a heterocyclic amino group having from 2 to 20 carbon atoms. Examples of the substituent include an alkyl group, a halogen atom and an ionic hydrophilic group.

The acylamino group includes a substituted acylamino group. The acylamino group is preferably an acylamino group having from 2 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the acylamino group include an acetylamino group, a propionylamino group, a benzoylamino group, an N-phenylacetylamino group and a 3,5-disulfobenzoylamino group.

The ureido group includes a substituted ureido group. The ureido group is preferably a ureido group having from 1 to 20 carbon atoms. Examples of the substituent include an alkyl group and an aryl group. Examples of the ureido group include a 3-methylureido group, a 3,3-dimethylureido group and a 3-phenylureido group.

The sulfamoylamino group includes a substituted sulfamoylamino group. Examples of the substituent include an alkyl group. Examples of the sulfamoylamino group include an N,N-dipropylsulfamoylamino group.

The alkoxycarbonylamino group includes a substituted alkoxycarbonylamino group. The alkoxycarbonylamino group is preferably an alkoxycarbonylamino group having from 2 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the alkoxycarbonylamino group include an ethoxycarbonylamino group.

The aryloxycarbonylamino group includes a substituted aryloxycarbonylamino group. The aryloxycarbonylamino group is preferably an aryloxycarbonylamino group having from 7 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the aryloxycarbonylamino group include a phenoxycarbonylamino group.

The alkylsulfonylamino group and the arylsulfonylamino group include a substituted alkylsulfonylamino group and a substituted arylsulfonylamino group. The alkylsulfonylamino group and the arylsulfonylamino group are preferably an alkylsulfonylamino group having from 1 to 20 carbon atoms and an arylsulfonylamino group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the alkylsulfonylamino group and arylsulfonylamino group include a methylsufonylamino group, an N-phenyl-methylsulfonylamino group, a phenylsulfonylamino group and a 3-carboxyphenylsulfonylamino group.

The heterocyclic sulfonylamino group includes a substituted heterocyclic sulfonylamino group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic sulfonylamino group is preferably a heterocyclic sulfonylamino group having from 1 to 12 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the heterocyclic sulfonylamino group include a 2-thienylsulfonylamino group and a 3-pyridylsulfonylamino group.

The alkylthio group, the arylthio group and the heterocyclic thio group include a substituted alkylthio group, a substituted arylthio group and a substituted heterocyclic thio group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The alkylthio group, the arylthio group and the heterocyclic thio group are preferably an alkylthio group having from 1 to 20 carbon atoms, an arylthio group having from 1 to 20 carbon atoms and a heterocyclic thio group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the alkylthio group, arylthio group and heterocyclic thio group include a methylthio group, a phenylthio group and a 2-pyridylthio group.

The alkylsulfonyl group and the arylsulfonyl group include a substituted alkylsulfonyl group and a substituted arylsulfonyl group. Examples of the alkylsulfonyl group and arylsulfonyl group include a methylsulfonyl group and a phenylsulfonyl group.

The heterocyclic sulfonyl group includes a substituted heterocyclic sulfonyl group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic sulfonyl group is preferably a heterocyclic sulfonyl group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the heterocyclic sulfonyl group include a 2-thienylsulfonyl group and a 3-pyridylsulfonyl group.

The alkylsulfinyl group and the arylsulfinyl group include a substituted alkylsulfinyl group and a substituted arylsulfinyl group. Examples of the alkylsulfinyl group and arylsulfinyl group include a methylsulfinyl group and a phenylsulfinyl group.

The heterocyclic sulfinyl group includes a substituted heterocyclic sulfinyl group. Examples of the heterocyclic ring include the heterocyclic rings described above for the heterocyclic group. The heterocyclic sulfinyl group is preferably a heterocyclic sulfinyl group having from 1 to 20 carbon atoms. Examples of the substituent include an ionic hydrophilic group. Examples of the heterocyclic sulfinyl group include a 4-pyridylsulfinyl group.

The sulfamoyl group includes a substituted sulfamoyl group. Examples of the substituent include an alkyl group. Examples of the sulfamoyl group include a dimethylsulfamoyl group and a di-(2-hydroxyethyl)sulfamoyl group.

The dye of formula (1) for use in the ink set of the present invention particularly preferably has a structure represented by the following formula (2):

In formula (2), R¹, R², R⁵ and R⁶ have the same meanings as in formula (1).

R³ and R⁴ each independently represents a hydrogen atom or a substituent, and the substituent is an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an alkylsulfonyl group, an arylsulfonyl group or a sulfamoyl group. R³ and R⁴ each is preferably a hydrogen atom, an aromatic group, a heterocyclic group, an acyl group, an alkylsulfonyl group or an arylsulfonyl group, more preferably a hydrogen atom, an aromatic group or a heterocyclic group.

Z¹ represents an electron withdrawing group having a Hammett's substituent constant σp value of 0.20 or more. Z¹ is preferably an electron withdrawing group having a up value of 0.30 or more, more preferably 0.45 or more, still more preferably 0.60 to more, but the σp value preferably does not exceed 1.0. Specific preferred examples of this substituent include electron withdrawing substituents described later. Among those, preferred are an acyl group having from 2 to 20 carbon atoms, an alkyloxycarbonyl group having from 2 to 20 carbon atoms, a nitro group, a cyano group, an alkylsulfonyl group having from 1 to 20 carbon atoms, an arylsulfonyl group having from 6 to 20 carbon atoms, a carbamoyl group having from 1 to 20 carbon atoms and a halogenated alkyl group having from 1 to 20 carbon atoms, more preferred are a cyano group, an alkylsulfonyl group having from 1 to 20 carbon atoms and an arylsulfonyl group having from 6 to 20 carbon atoms, and most preferred is a cyano group.

Z² represents a hydrogen atom or a substituent and the substituent is an aliphatic group, an aromatic group or a heterocyclic group. Z² is preferably an aliphatic group, more preferably an alkyl group having from 1 to 6 carbon atoms.

Q represents a hydrogen atom or a substituent and the substituent is an aliphatic group, an aromatic group or a heterocyclic group. Q is preferably a group comprising a nonmetallic atom group necessary for forming a 5-, 6-, 7- or 8-membered ring. The 5-, 6-, 7- or 8-membered ring may be substituted, may be a saturated ring or may have an unsaturated bond. Q is more preferably an aromatic group or a heterocyclic group. Preferred examples of the nonmetallic atom include a nitrogen atom, an oxygen atom, a sulfur atom and a carbon atom. Specific examples of the ring structure include a benzene ring, a cyclopentane ring, a cyclohexane ring, a cycloheptane ring, a cyclooctane ring, a cyclohexene ring, a pyridine ring, a pyrimidine ring, a pyrazine ring, a pyridazine ring, a triazine ring, an imidazole ring, a benzimidazole ring, an oxazole ring, a benzoxazole ring, a thiazole ring, a benzothiazole ring, an oxane ring, a sulfolane ring and a thiane ring.

The hydrogen atom of each substituent described in regard to formula (2) may be substituted. Examples of the substituent include the substituents described in regard to formula (1), the groups described as examples for G, R¹ and R², and ionic hydrophilic groups.

Here, the Hammett's substituent constant σp value used in the present invention is described. The Hammett's rule is a rule of thumb advocated by L. P. Hammett in 1935 so as to quantitatively discuss the effect of substituent on the reaction or equilibrium of benzene derivatives and its propriety is widely admitted at present. The substituent constant determined by the Hammett's rule includes a σp value and a σm value and these values can be found in a large number of general publications but these are described in detail, for example, in J. A. Dean (compiler), Lange's Handbook of Chemistry, 12th ed., McGraw-Hell (1979), and Kagakuno Ryoiki (Chemistry Region), special number, No. 122, pp. 96-103, Nankodo (1979). In the present invention, each substituent is limited or described using the Hammett's substituent constant σp value but this does not mean that the substituent is limited only to those having a known value which can be found in the above-described publications. Needless to say, the substituent includes substituents of which σp value is not known in publications but when measured based on the Hammett's rule, falls within the range specified. Furthermore, although formula (2) of the present invention includes those which are not a benzene derivative, the σp value is used as a measure for showing the electron effect of the substituent irrespective of the substitution site. In the present invention, the σp value is used in this meaning.

Examples of the electron withdrawing group having a Hammett's substituent constant σp value of 0.60 or more include a cyano group, a nitro group, an alkylsulfonyl group (e.g., methylsulfonyl) and an arylsulfonyl group (e.g., phenylsulfonyl).

Examples of the electron withdrawing group having a Hammett's σp value of 0.45 or more include, in addition to those described above, an acyl group (e.g., acetyl), an alkoxycarbonyl group (e.g., dodecyloxycarbonyl), an aryloxycarbonyl group (e.g., m-chlorophenoxycarbonyl), an alkylsulfinyl group (e.g., n-propylsulfinyl), an arylsulfinyl group (e.g., phenylsulfinyl), a sulfamoyl group (e.g., N-ethylsulfamoyl, N,N-dimethylsulfamoyl) and a halogenated alkyl group (e.g., trifluoromethyl).

Examples of the electron withdrawing group having a Hammett's substituent constant σp value of 0.30 or more include, in addition to those described above, an acyloxy group (e.g., acetoxy), a carbamoyl group (e.g., N-ethylcarbamoyl, N,N-dibutylcarbamoyl), a halogenated alkoxy group (e.g., triphloromethyloxy), a halogenated aryloxy group (e.g., pentaphlorophenyloxy), a sulfonyloxy group (e.g., methylsulfonyloxy), a halogenated alkylthio group (e.g., diphloromethylthio), an aryl group substituted by two or more electron withdrawing groups having a σp value of 0.15 or more (e.g., 2,4-dinitrophenyl, pentachlorophenyl) and a heterocyclic ring (e.g., 2-benzoxazolyl, 2-benzothiazolyl, 1-phenyl-2-benzimidazolyl).

Specific examples of the electron withdrawing group having a σp value of 0.20 or more include, in addition to those described above, a halogen atom.

In the azo dye represented by formula (1), the preferred combination of substituents is described below. R⁵ and R⁶ each is preferably a hydrogen atom, an alkyl group, an aryl group, a heterocyclic group, a sulfonyl group or an acyl group, more preferably a hydrogen atom, an aryl group, a heterocyclic group or a sulfonyl group, and most preferably a hydrogen atom, an aryl group or a heterocyclic group. However, R⁵ and R⁶ are not a hydrogen atom at the same time.

G is preferably a hydrogen atom, a halogen atom, an alkyl group, a hydroxyl group, an amino group or an acylamino group, more preferably a hydrogen atom, a halogen atom, an amino group or an acylamino group, and most preferably a hydrogen atom, an amino group or an acylamino group.

A is preferably a pyrazole ring, an imidazole ring, an isothiazole ring, a thiadiazole ring or a benzothiazole ring, more preferably a pyrazole ring or an isothiazole ring, and most preferably a pyrazole ring.

B¹ and B² each is ═CR¹— or —CR²═, and R¹ and R² each is preferably a hydrogen atom, an alkyl group, a halogen atom, a cyano group, a carbamoyl group, a carboxyl group, a hydroxyl group, an alkoxy group or an alkoxycarbonyl group, more preferably a hydrogen atom, an alkyl group, a carboxyl group, a cyano group or a carbamoyl group.

As for the preferred combination of substituents in the compound represented by formula (1), a compound where at least one of various substituents is the preferred group is preferred, a compound where a larger number of various substituents are the preferred groups is more preferred, and a compound where all substituents are the preferred groups is most preferred.

Specific examples of the azo dye represented by formula (1) are set forth below, however, the azo dye for use in the present invention is not limited to those set forth below.

R₁ R₂ R₃ a-1

a-2

a-3

a-4

a-5

a-6

a-7

a-8

a-9

C₈H₁₇(t) a-10

R₁ R₂ R₃ R₄ a-11

a-12

a-13

a-14

a-15

a-16

a-17

R₁ R₂ R₃ R₄ a-18

a-19

—SO₂CH₃

a-20

—COCH₃ C₈H₁₇(t) C₈H₁₇(t) a-21

—SO₂CH₃

C₈H₁₇(t) a-22

H

a-23

H

a-24

H

a-25

a-26

a-27

a-28

a-29

a-30

C₈H₁₇(t) a-31

a-32

a-33

a-34

a-35

a-36

a-37

a-38

a-39

a-40

R₁ R₂ R₃ R₄ R₅ R₆ R₇ R₈ a-41

CN

H CONH₂ SO₂CH₃

a-42

Br

COOEt H

C₈H₁₇(t) COCH₃ a-43

SO₂CH₃

CONH₂ H

a-44

CN

H H

SO₂CH₂ a-45

Br

H CONH₂

a-46

CN

CH₃ H

R₁ R₂ R₃ R₄ R₅ R₆ b-1 CH₃ CH₃ CN H

b-2 CH₃ CH₃ CN H

b-3 CH₃ CH₃ CONH₂ H

b-4 CH₃ CH₃ H H

b-5 CH₃ H CN H

b-6 CH₃ CH₃ H

b-7 CH₃ CH₃ H

b-8 CH₃ H H SO₂CH₃

R₁ R₂ R₃ R₄ R₅ R₆ c-1 —SCH₃ CH₃ CN H C₈H₁₇(t)

c-2

H CONH₂ H

c-3

CH₃ H

c-4 CH₃ CH₃ H

c-5

H H

C₈H₁₇(t)

R₁ R₂ R₃ R₄ R₅ R₆ d-1 Me CH₃ CN H

d-2 Me CH₃ CN H

d-3 Me H H

d-4 Ph CH₃ CONH₂ H

d-5 Ph CH₃ H

R₁ R₂ R₃ R₄ R₅ R₆ e-1 5-Cl CH₃ CONH₂ H C₈H₁₇(t) C₈H₁₇(t) e-2 5,6-diCl H H

e-3 5,6-diCl CH₃ H

COCH₃ e-4 5-CH₃ H CN H

e-5 5-NO₂ CH₃ H SO₂CH₃

f-1

f-2

The cyan dye which can be preferably used in a cyan or light cyan ink or a dark yellow ink of the ink set for inkjet use of the present invention and is effective in maintaining the image fastness and color balance, is preferably a dye represented by the following formula (I):

wherein X₁, X₂, X₃ and X₄ each independently represents —SO-Z, —SO₂-Z, —SO₂NR₁R₂, a sulfo group, —CONR₁R₂ or —CO₂R₁; Z represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; R₁ and R₂ each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group; when a plural number of Zs are present, the plurality of Zs may be the same or different; Y₁, Y₂, Y₃ and Y₄ each independently represents a monovalent substituent; when a plural number of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s are present, the plurality of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s may be the same or different; M represents a hydrogen atom, a metal atom or an oxide, hydroxide or halide thereof; a₁ to a₄ and b₁ to b₄ each represents the number of substituent X₁, X₂, X₃, X₄, Y₁, Y₂, Y₃ or Y₄; a₁ to a₄ each independently represents an integer of 0 to 4 but all are not 0 at the same time; and b₁ to b₄ each independently represents an integer of 0 to 4.

The compound of formula (I) for use in the present invention is described in more detail below.

In formula (I), X₁, X₂, X₃ and X₄ each independently represents —SO-Z, —SO₂-Z, —SO₂NR₁R₂, a sulfo group, —CONR₁R₂ or —CO₂R₁. Among these substituents, preferred are —SO-Z, —SO₂-Z, —SO₂NR₁R₂ and —CONR₁R₂, more preferred are —SO₂-Z and —SO₂NR₁R₂, and most preferred is —SO₂-Z. In the case where any one of a₁ to a₄ each showing the number of substituent represents a number of 2 or more, the plurality of X₁s, X₂s, X₃s or X₄s may be the same or different and each independently represents any one of the above-described groups. X₁, X₂, X₃ and X₄ may be completely the same substituents, may be substituents of the same kind but partially different, for example, as in the case where X₁, X₂, X₃ and X₄ all are —SO₂-Z and Zs are different from each other, or may include substituents different from each other, for example, —SO₂-Z and —SO₂NR₁R₂.

Each Z independently represents a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, preferably a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group, and most preferably a substituted alkyl group, a substituted aryl group or a substituted heterocyclic group.

R₁ and R₂ each independently represents a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted aralkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, preferably a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, more preferably a hydrogen atom, a substituted alkyl group, a substituted aryl group or a substituted heterocyclic group. However, it is not preferred that R₁ and R₂ both are a hydrogen atom.

The substituted or unsubstituted alkyl group represented by R₁, R₂ and Z is preferably an alkyl group having from 1 to 30 carbon atoms, more preferably a branched alkyl group because the solubility of dye and the stability of ink are improved, and still more preferably an alkyl group having an asymmetric carbon (use in the racemic form). Examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. In particular, a hydroxyl group, an ether group, an ester group, a cyano group, an amido group and a sulfonamido group are preferred because the aggregating property and fastness of dye are enhanced. Other than these, the alkyl group may be substituted by a halogen atom or an ionic hydrophilic group. Incidentally, the number of carbon atoms in the alkyl group does not contain carbon atoms of substituents and this applies to other groups.

The substituted or unsubstituted cycloalkyl group represented by R₁, R₂ and Z is preferably a cycloalkyl group having from 5 to 30 carbon atoms, more preferably a cycloalkyl group having an asymmetric carbon (use in the racemic form) because the solubility of dye and the stability of ink are improved. Examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. In particular, a hydroxyl group, an ether group, an ester group, a cyano group, an amido group and a sulfonamido group are preferred because the aggregating property and fastness of dye are enhanced. Other than these, the cycloalkyl group may be substituted by a halogen atom or an ionic hydrophilic group.

The substituted or unsubstituted alkenyl group represented by R₁, R₂ and Z is preferably an alkenyl group having from 2 to 30 carbon atoms, more preferably a branched alkenyl group because the solubility of dye and the stability of ink are improved, and still more preferably an alkenyl group having an asymmetric carbon (use in the racemic form). Examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. In particular, a hydroxyl group, an ether group, an ester group, a cyano group, an amido group and a sulfonamido group are preferred because the aggregating property and fastness of dye are enhanced. Other than these, the alkenyl group may be substituted by a halogen atom or an ionic hydrophilic group.

The substituted or unsubstituted aralkyl group represented by R₁, R₂ and Z is preferably an aralkyl group having from 7 to 30 carbon atoms, more preferably a branched aralkyl group because the solubility of dye and the stability of ink are improved, and still more preferably an aralkyl group having an asymmetric carbon (use in the racemic form). Examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. In particular, a hydroxyl group, an ether group, an ester group, a cyano group, an amido group and a sulfonamido group are preferred because the aggregating property and fastness of dye are enhanced. Other than these, the aralkyl group may be substituted by a halogen atom or an ionic hydrophilic group.

The substituted or unsubstituted aryl group represented by R₁, R₂ and Z is preferably an aryl group having from 6 to 30 carbon atoms. Examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. In particular, an electron withdrawing group is preferred because the dye can be noble in the oxidation potential and improved in the fastness. Examples of the electron withdrawing group include those having a positive Hammett's substituent constant σp value. Among these, preferred are a halogen atom, a heterocyclic group, a cyano group, a carboxyl group, an acylamino group, a sulfonamido group, a sulfamoyl group, a carbamoyl group, a sulfonyl group, an imido group, an acyl group, a sulfo group and a quaternary ammonium group, more preferred are a cyano group, a carboxyl group, a sulfamoyl group, a carbamoyl group, a sulfonyl group, an imido group, an acyl group, a sulfo group and a quaternary ammonium group.

The heterocyclic group represented by R₁, R₂ and Z is preferably a 5- or 6-membered ring and the ring may be further condensed. Also, the ring may be an aromatic heterocyclic ring or a non-aromatic heterocyclic ring. Examples of the heterocyclic group represented by R₁, R₂ and Z are shown below in the form of a heterocyclic ring by omitting the substitution site. The substitution site is not limited and for example, pyridine may be substituted at the 2-position, 3-position and 4-position. Examples include pyridine, pyrazine, pyrimidine, pyridazine, triazine, quinoline, isoquinoline, quinazoline, cinnoline, phthalazine, quinoxaline, pyrrole, indole, furan, benzofuran, thiophene, benzothiophene, pyrazole, imidazole, benzimidazole, triazole, oxazole, benzoxazole, thiazole, benzothiazole, isothiazole, benzisothiazole, thiadiazole, isoxazole, benzisoxazole, pyrrolidine, piperidine, piperazine, imidazolidine and thiazoline. In particular, an aromatic heterocyclic group is preferred. Preferred examples thereof include, shown in the same manner as above, pyridine, pyrazine, pyrimidine, pyridazine, triazine, pyrazole, imidazole, benzimidazole, triazole, thiazole, benzothiazole, isothiazole, benzisothiazole and thiadiazole. These groups each may have a substituent and examples of the substituent include those described later as the substituent when Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ can further have a substituent. Preferred substituents are the same as the above-described substituents of the aryl group and more preferred substituents are the same as the above-described more preferred substituents of the aryl group.

Y₁, Y₂, Y₃ and Y₄ each independently represents a hydrogen atom, a halogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group, a heterocyclic group, a cyano group, a hydroxyl group, a nitro group, an amino group, an alkylamino group, an alkoxy group, an aryloxy group, an acylamino group, an arylamino group, a ureido group, a sulfamoylamino group, an alkylthio group, an arylthio group, an alkoxycarbonylamino group, a sulfonamido group, a carbamoyl group, a sulfamoyl group, a sulfonyl group, an alkoxycarbonyl group, a heterocyclic oxy group, an azo group, an acyloxy group, a carbamoyloxy group, a silyloxy group, an aryloxycarbonyl group, an aryloxycarbonylamino group, an imido group, a heterocyclic thio group, a phosphoryl group, an acyl group, a carboxyl group or a sulfo group. These groups each may further have a substituent.

Among these, preferred are a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a cyano group, an alkoxy group, an amido group, a ureido group, a sulfonamido group, a carbamoyl group, a sulfamoyl group, an alkoxycarbonyl group, a carboxyl group and a sulfo group, more preferred are a hydrogen atom, a halogen atom, a cyano group, a carboxyl group and a sulfo group, and most preferred is a hydrogen atom.

When Z, R₁, R₂, Y₁, Y₂, Y₃ and Y₄ each is a group which can further have a substituent, the group may further have the following substituent.

Examples of the substituent include a linear or branched alkyl group having from 1 to 12 carbon atoms, a linear or branched aralkyl group having from 7 to 18 carbon atoms, a linear or branched alkenyl group having from 2 to 12 carbon atoms, a linear or branched alkynyl group having from 2 to 12 carbon atoms, a linear or branched cycloalkyl group having from 3 to 12 carbon atoms, a linear or branched cycloalkenyl group having from 3 to 12 carbon atoms (these groups each is preferably a group having a branched chain because the solubility of dye and the stability of ink are improved, more preferably a group having an asymmetric carbon; specific examples of the groups include a methyl group, an ethyl group, a propyl group, an isopropyl group, a sec-butyl group, a tert-butyl group, a 2-ethylhexyl group, a 2-methylsulfonylethyl group, a 3-phenoxypropyl group, a trifluoromethyl group and a cyclopentyl group), a halogen atom (e.g., chlorine, bromine), an aryl group (e.g., phenyl, 4-tert-butylphenyl, 2,4-di-tert-amylphenyl), a heterocyclic group (e.g., imidazolyl, pyrazolyl, triazolyl, 2-furyl, 2-thienyl, 2-pyrimidinyl, 2-benzothiazolyl),

-   a cyano group, a hydroxyl group, a nitro group, a carboxy group, an     amino group, an alkyloxy group (e.g., methoxy, ethoxy,     2-methoxyethoxy, 2-methanesulfonylethoxy), an aryloxy group (e.g.,     phenoxy, 2-methylphenoxy, 4-tert-butylphenoxy, 3-nitrophenoxy,     3-tert-butyloxycarbamoyl-phenoxy, 3-methoxycarbamoyl), an acylamino     group (e.g., acetamido, benzamido,     4-(3-tert-butyl-4-hydroxyphenoxy)-butanamido), an alkylamino group     (e.g., methylamino, butylamino, diethylamino, methylbutylamino), an     anilino group (e.g., phenylamino, 2-chloroanilino), a ureido group     (e.g., phenylureido, methylureido, N,N-dibutylureido), a     sulfamoylamino group (e.g., N,N-dipropylsulfamoylamino), an     alkylthio group (e.g., methylthio, octylthio, 2-phenoxyethylthio),     an arylthio group (e.g., phenylthio,     2-butoxy-5-tert-octylphenylthio, 2-carboxyphenylthio), an     alkyloxycarbonylamino group (e.g., methoxycarbonylamino), a     sulfonamido group (e.g., methanesulfonamido, benzenesulfonamido,     p-toluenesulfonamido), -   a carbamoyl group (e.g., N-ethylcarbamoyl, N,N-dibutylcarbamoyl), a     sulfamoyl group (e.g., N-ethylsulfamoyl, N,N-dipropylsulfamoyl,     N-phenylsulfamoyl), a sulfonyl group (e.g., methanesulfonyl,     octanesulfonyl, benzenesulfonyl, toluenesulfonyl), an     alkyloxycarbonyl group (e.g., methoxycarbonyl, butyloxycarbonyl), a     heterocyclic oxy group (e.g., 1-phenyltetrazol-5-oxy,     2-tetrahydropyranyloxy), an azo group (e.g., phenylazo,     4-methoxyphenylazo, 4-pivaloylaminophenylazo,     2-hydroxy-4-propanoylphenylazo), an acyloxy group (e.g., acetoxy), a     carbamoyloxy group (e.g., N-methylcarbamoyloxy,     N-phenylcarbamoyloxy), -   a silyloxy group (e.g., trimethylsilyloxy, dibutylmethylsilyloxy),     an aryloxycarbonylamino group (e.g., phenoxycarbonylamino), an imido     group (e.g., N-succinimido, N-phthalimido), a heterocyclic thio     group (e.g., 2-benzothiazolylthio,     2,4-di-phenoxy-1,3,5-triazole-6-thio, 2-pyridylthio), a sulfinyl     group (e.g., 3-phenoxypropylsulfinyl), a phosphonyl group (e.g.,     phenoxyphosphonyl, octyloxyphosphonyl, phenylphosphonyl), an     aryloxycarbonyl group (e.g., phenoxycarbonyl), an acyl group (e.g.,     acetyl, 3-phenylpropanoyl, benzoyl), and an ionic hydrophilic group     (e.g., carboxyl, sulfo, phosphono, quaternary ammonium).

In the case where the phthalocyanine dye represented by formula (I) is water-soluble, the dye preferably has an ionic hydrophilic group. Examples of the ionic hydrophilic group include a sulfo group, a carboxyl group, a phosphono group and a quaternary ammonium group. Among these ionic hydrophilic groups, preferred are a carboxyl group, a phosphono group and a sulfo group, more preferred are a carboxyl group and a sulfo group. The carboxyl group, the phosphono group and the sulfo group each may be in the form of a salt and examples of the counter ion for forming the salt include ammonium ion, alkali metal ion (e.g., lithium ion, sodium ion, potassium ion) and organic cation (e.g., tetramethylammonium ion, tetramethylguanidium ion, tetramethylphosphonium). Among these counter ions, an alkali metal salt is preferred and a lithium salt is more preferred because this salt enhances the solubility of dye and the stability of ink.

As for the number of ionic hydrophilic groups, the phthalocyanine dye preferably has at least two ionic hydrophilic groups, more preferably at least two sulfo groups and/or carboxyl groups, within one molecule.

a₁ to a₄ and b₁ to b₄ each represents the number of substituent X₁, X₂, X₃, X₄, Y₁, Y₂, Y₃ or Y₄. a₁ to a₄ each independently represents an integer of 0 to 4 but all are not 0 at the same time. b₁ to b₄ each independently represents an integer of 0 to 4. When any one of a₁ to a₄ and b₁ to b₄ represents an integer of 2 or more, a plural number of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s are present and the plurality of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s may be the same or different.

a₁ and b₁ satisfy the relationship of a₁+b₁=4. In particular, a combination that a₁ represents 1 or 2 and b₁ represents 3 or 2 is preferred, and a combination that a₁ represents 1 and b₁ represents 3 is most preferred.

The same relationship as that between a₁ and b₁ is present in each of the pairs a₂ and b₂, a₃ and b₃, and a₄ and b₄, and the preferred combination is also the same.

M represents a hydrogen atom, a metal element or an oxide, hydroxide or halide thereof.

M is preferably a hydrogen atom, a metal element such as Li, Na, K, Mg, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Si, Ge, Sn, Pb, Sb and Bi, an oxide such as VO and GeO, a hydroxide such as Si(OH)₂, Cr(OH)₂ and Sn(OH)₂, or a halide such as AlCl, SiCl₂, VCl, VCl₂, VOCl, FeCl, GaCl and ZrCl. Among these, more preferred are Cu, Ni, Zn and Al, and most preferred is Cu.

Also, Pc (phthalocyanine ring) may form a dimer (for example, Pc-M-L-M-Pc) or a trimer through L (divalent linking group). At this time, Ms may be the same or different.

The divalent linking group represented by L is preferably an oxy group —O—, a thio group —S—, a carbonyl group —CO—, a sulfonyl group —SO₂—, an imino group —NH—, a methylene group —CH₂— or a group formed by combining two or more of these groups.

As for the preferred combination of substituents in the compound represented by formula (I), a compound where at least one of various substituents is the preferred group is preferred, a compound where a larger number of various substituents are the preferred groups is more preferred, and a compound where all substituents are the preferred groups is most preferred.

Among the phthalocyanine dyes represented by formula (I), a phthalocyanine dye having a structure represented by formula (II) is preferred. The phthalocyanine dye represented by formula (II), which is particularly suitable for the present invention, is described in detail below.

In formula (II), X₁₁ to X₁₄ and Y₁₁ to Y₁₈ have the same meanings as X₁ to X₄ and Y₁ to Y₄ in formula (I), respectively, and preferred examples are also the same. M has the same meaning as M in formula (I) and preferred examples are also the same.

In formula (II), a₁₁ to a₁₄ each independently represents an integer of 1 or 2. a₁₁+a₁₂+a₁₃+a₁₄ is preferably from 4 to 6 and a₁₁=a₁₂=a₁₃=a₁₄=1 is more preferred.

X₁₁, X₁₂, X₁₃ and X₁₄ may be completely the same substituents, may be substituents of the same kind but partially different, for example, as in the case where X₁₁, X₁₂, X₁₃ and X₁₄ all are —SO₂-Z and Zs are different from each other, or may include substituents different from each other, for example, —SO₂-Z and —SO₂NR₁R₂.

In the phthalocyanine dye represented by formula (II), the following combination of substituents is particularly preferred.

X₁₁ to X₁₄ each independently represents preferably —SO-Z, —SO₂-Z, —SO₂NR₁R₂ or —CONR₁R₂, more preferably —SO₂Z or —SO₂NR₁R₂, most preferably —SO₂-Z.

Each Z independently represents preferably a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group, and most preferably a substituted alkyl group, a substituted aryl group or a substituted heterocyclic group. In particular, the case where an asymmetric carbon is present in the substituent (use in the racemic form) is preferred because the solubility of dye and the stability of ink are enhanced. Also, the case where a hydroxyl group, an ether group, an ester group, a cyano group, an amido group or a sulfonamido group is present in the substituent is preferred because the aggregating property and fastness are improved.

R₁ and R₂ each independently represents preferably a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group or a substituted or unsubstituted heterocyclic group, and most preferably a hydrogen atom, a substituted alkyl group, a substituted aryl group or a substituted heterocyclic group. However, it is not preferred that R₁ and R₂ both are a hydrogen atom. In particular, the case where an asymmetric carbon is present in the substituent (use in the racemic form) is preferred because the solubility of dye and the stability of ink are enhanced. Also, the case where a hydroxyl group, an ether group, an ester group, a cyano group, an amido group or a sulfonamido group is present in the substituent is preferred because the aggregating property and fastness are improved.

Y₁₁ to Y₁₈ each independently represents preferably a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a cyano group, an alkoxy group, an amido group, a ureido group, a sulfonamido group, a carbamoyl group, a sulfamoyl group, an alkoxycarbonyl group, a carboxyl group or a sulfo group, more preferably a hydrogen atom, a halogen atom, a cyano group, a carboxyl group or a sulfo group, and most preferably a hydrogen atom.

a₁₁ to a₁₄ each independently represents preferably 1 or 2 and it is particularly preferred that all are 1.

M represents a hydrogen atom, a metal element or an oxide, hydroxide or halide thereof, more preferably Cu, Ni, Zn or Al, and most preferably Cu.

In the case where the phthalocyanine dye represented by formula (II) is water-soluble, the dye preferably has an ionic hydrophilic group. Examples of the ionic hydrophilic group include a sulfo group, a carboxyl group, a phosphono group and a quaternary ammonium group. Among these ionic hydrophilic groups, preferred are a carboxyl group, a phosphono group and a sulfo group, more preferred are a carboxyl group and a sulfo group. The carboxyl group, the phosphono group and the sulfo group each may be in the form of a salt and examples of the counter ion for forming the salt include ammonium ion, alkali metal ion (e.g., lithium ion, sodium ion, potassium ion) and organic cation (e.g., tetramethylammonium ion, tetramethylguanidium ion, tetramethylphosphonium). Among these counter ions, alkali metal ion is preferred and lithium ion is more preferred because this enhances the solubility of dye and improves the stability of ink.

As for the number of ionic hydrophilic groups, the phthalocyanine dye preferably has at least two ionic hydrophilic groups, more preferably at least two sulfo groups and/or carboxyl groups, within one molecule.

As for the preferred combination of substituents in the compound represented by formula (II), a compound where at least one of various substituents is the preferred group is preferred, a compound where a larger number of various substituents are the preferred groups is more preferred, and a compound where all substituents are the preferred groups is most preferred.

As for the chemical structure of the phthalocyanine dye of the present invention, at least one electron withdrawing group such as sulfinyl group, sulfonyl group and sulfamoyl group is preferably introduced into respective four benzene rings of phthalocyanine such that the total of σp values of the substituents in the entire phthalocyanine skeleton becomes 1.6 or more.

The Hammett's substituent constant σp value is briefly described. The Hammett's rule is a rule of thumb advocated by L. P. Hammett in 1935 so as to quantitatively discuss the effect of substituent on the reaction or equilibrium of benzene derivatives and its propriety is widely admitted at present. The substituent constant determined by the Hammett's rule includes a σp value and a σm value and these values can be found in a large number of general publications but these are described in detail, for example, in J. A. Dean (compiler), Lange's Handbook of Chemistry, 12th ed., McGraw-Hell (1979), and Kagakuno Ryoiki (Chemistry Region), special number, No. 122, pp. 96-103, Nankodo (1979).

Generally, in view of the synthesis method, the phthalocyanine derivative represented by formula (I) is inevitably a mixture of analogues differing in the site where the substituents Xn (n=1 to 4) and Ym (m=1 to 4) are introduced and in the number of the substituents introduced. Accordingly, these analogue mixtures are statistically averaged and represented by a formula in many cases. In the present invention, it has been found that when these analogue mixtures are classified into the following three types, a specific mixture is particularly preferred. The phthalocyanine-base dye analogue mixtures represented by formulae (I) and (II) are defined by classifying these into the following three types based on the substitution site.

(1) β-Position Substitution Type:

A phthalocyanine dye having specific substituents at the 2- and/or 3-position, the 6- and/or 7-position, the 10- and/or 11-position, and the 14- and/or 15-position.

(2) α-Position Substitution Type:

A phthalocyanine dye having specific substituents at the 1- and/or 4-position, the 5- and/or 8-position, the 9- and/or 12-position, and the 13- and/or 16-position.

(3) α,β-Position Mixed Substitution Type:

A phthalocyanine dye having specific substitutions at the 1- to 16-positions without any regularity.

In the present invention, phthalocyanine dye derivatives differing in the structure (particularly in the substitution site) are described by using these β-position substitution type, α-position substitution type and α,β-position mixed substitution type.

The phthalocyanine derivative for use in the present invention can be synthesized by combining the methods described or cited, for example, in Shirai and Kobayashi, Phthalocyanine—Kagaku to Kino—(Phthalocyanine—Chemistry and Function—), pp. 1-62, IPC, and C. C. Leznoff and A. B. P. Lever, Phthalocyanines—Properties and Applications, pp. 1-54, VCH, or methods analogous thereto.

The phthalocyanine compound represented by formula (I) of the present invention can be synthesized, for example, through sulfonation, sulfonyl chloridation and amidation of an unsubstituted phthalocyanine compound as described in International Patents 00/17275, 00/08103, 00/08101 and 98/41853 and JP-A-10-36471. In this case, sulfonation may take place at any site of the phthalocyanine nucleus and the number of sites sulfonated is difficult to control. Accordingly, when a sulfo group is introduced under such reaction conditions, the site and number of sulfo groups introduced into the product cannot be specified and a mixture of those differing in the number of substituents or in the substitution site is inevitably produced. If the compound of the present invention is synthesized starting from such a product, the compound of the present invention is obtained as an α,β-position mixed substitution type mixture containing several kinds of compounds differing in the number of substituents or in the substitution site because the number of sulfamoyl groups substituted on the heterocyclic ring or their substitution sites cannot be specified.

As described above, for example, when many electron withdrawing groups such as sulfamoyl group are introduced into the phthalocyanine nucleus, the oxidation potential becomes nobler and the ozone resistance is increased. However, according to the above-described synthesis method, a phthalocyanine dye where the number of electron withdrawing groups introduced is small, namely, the oxidation potential is baser, is inevitably mingled. Accordingly, in order to improve the ozone resistance, it is preferred to use a synthesis method where the production of a compound having a baser oxidation potential is suppressed.

The phthalocyanine compound represented by formula (II) of the present invention can be synthesized, for example, by reacting a phthalonitrile derivative (Compound P) shown below and/or a diiminoisoindoline derivative (Compound Q) shown below with a metal derivative represented by formula (III) or may be derived from a tetrasulfophthalocyanine compound obtained by reacting a 4-sulfophthalonitrile derivative (Compound R) shown below with a metal derivative represented by formula (III).

In the formulae above, X_(p) corresponds to X₁₁, X₁₂, X₁₃ or X₁₄ in formula (II) and Y_(q) and Y_(q′) each corresponds to Y₁₁, Y₁₂, Y₁₃, Y₁₄, Y₁₅, Y₁₆, Y₁₇ or Y₁₈ in formula (II). In Compound R, M′ represents cation.

Examples of the cation represented by M′ include alkali metal ion such as Li, Na and K, and organic cation such as triethylammonium ion and pyridinium ion. Formula (III): M-(Y)_(d) wherein M has the same meaning as M in formulae (I) and (II), Y represents a monovalent or divalent ligand such as halogen atom, acetate anion, acetylacetonate and oxygen, and d represents an integer of 1 to 4.

That is, according to this synthesis method, a specific number of desired substituents can be introduced. Particularly, in the case of introducing a large number of electron withdrawing groups so as to render the oxidation potential nobler as in the present invention, this synthesis method is very excellent as compared with the method described above for synthesizing the phthalocyanine compound of formula (I).

The thus-obtained phthalocyanine compound represented by formulae (II) is usually a mixture of compounds represented by the following formulae (a)-1 to (a)-4 which are isomers in respect to the substitution site of each X_(p), namely, a β-position substitution type.

In the synthesis method above, when all Xp are the same, a β-position substitution type phthalocyanine dye where X₁₁, X₁₂, X₁₃ and X₁₄ are completely the same substituents can be obtained. On the other hand, when Xp are different, a dye having substituents of the same kind but partially different from each other or a dye having substituents different from each other can be synthesized. Among the dyes of formula (II), these dyes having electron withdrawing substituents different from each other are preferred because the solubility and aggregating property of dye and the aging stability of ink can be controlled.

In the present invention, it has been found to be very important for the improvement of fastness that in any substitution type, the oxidation potential is nobler than 1.0 V (vs SCE). The great effect thereof cannot be expected at all from the above-described known techniques. Furthermore, although the cause is not particularly known, there is a tendency that the β-position substitution type is apparently more excellent in the color hue, light fastness, ozone resistance and the like than the α,β-position mixed substitution type.

Specific examples (Compounds I-1 to I-12 coming under formula (I) and Compounds 101 to 190 coming under formula (II)) of the phthalocyanine dyes represented by formulae (I) and (II) are set forth below, however, the phthalocyanine dye for use in the present invention is not limited to the following examples.

In the following Tables, specific examples of each pair of (X₁, X₂), (Y₁₁, Y₁₂), (Y₁₃, Y₁₄), (Y₁₅, Y₁₆) and (Y₁₇, Y₁₈) are independently in an irregular order.

No. M X₁ X₂ Y₁₁, Y₁₂ Y₁₃, Y₁₄ Y₁₅, Y₁₆ Y₁₇, Y₁₈ 101 Cu —SO₂—NH—CH₂—CH₂—SO₃Li —H —H, —H —H, —H —H, —H —H, —H 102 Cu

—H —Cl, —H —Cl, —H —Cl, —H —Cl, —H 103 Cu

—H —H, —H —H, —H —H, —H —H, —H 104 Cu

—H —H, —H —H, —H —H, —H —H, —H 105 Ni

—H —Cl, —H —Cl, —H —Cl, —H —Cl, —H 106 Cu —SO₂—NH—CH₂—CH₂—SO₂—NH—CH₂—COONa —CN —H, —H —H, —H —H, —H —H, —H 107 Cu

—H —H, —H —H, —H —H, —H —H, —H 108 Cu SO₂—CH₂—CH₂—CH₂—SO₃Li —H —H, —H —H, —H —H, —H —H, —H 109 Cu SO₂—CH₂—CH₂—CH₂—SO₃K —H —H, —H —H, —H —H, —H —H, —H 110 Cu SO₂—(CH₂)₅—CO₂K —H —H, —H —H, —H —H, —H —H, —H 111 Cu

—H —H, —H —H, —H —H, —H —H, —H 112 Cu

—SO₂Li —H, —H —H, —H —H, —H —H, —H 113 Cu

—H —H, —H —H, —H —H, —H —H, —H 114 Cu

—SO₃Li —H, —H —H, —H —H, —H —H, —H 115 Cu

—H —H, —H —H, —H —H, —H —H, —H 116 Cu

—H —H, —H —H, —H —H, —H —H, —H 117 Cu

—H —H, —H —H, —H —H, —H —H, —H 118 Cu

—H —H, —H —H, —H —H, —H —H, —H 119 Cu

—H —H, —H —H, —H —H, —H —H, —H 120 Cu

—H —H, —H —H, —H —H, —H —H, —H 121 Cu

—H —H, —H —H, —H —H, —H —H, —H 122 Cu

—H —H, —H —H, —H —H, —H —H, —H 123 Cu —SO₂NH—CH₃H₁₇(t) —H —H, —H —H, —H —H, —H —H, —H 124 Cu

—H —H, —H —H, —H —H, —H —H, —H 125 Cu

—H —H, —H —H, —H —H, —H —H, —H 126 Cu

—H —H, —H —H, —H —H, —H —H, —H 127 Cu

—H —H, —H —H, —H —H, —H —H, —H 128 Zn

—CN —H, —H —H, —H —H, —H —H, —H 129 Cu

—H —H, —H —H, —H —H, —H —H, —H 130 Cu

—H —H, —H —H, —H —H, —H —H, —H 131 Cu

—H —H, —H —H, —H —H, —H —H, —H 132 Cu

—H —H, —H —H, —H —H, —H —H, —H 133 Cu

—H —H, —H —H, —H —H, —H —H, —H 134 Cu

—H —H, —H —H, —H —H, —H —H, —H 135 Cu

—H —H, —H —H, —H —H, —H —H, —H 136 Cu

—H —H, —H —H, —H —H, —H —H, —H 137 Cu

—H —H, —H —H, —H —H, —H —H, —H 138 Cu

—H —H, —H —H, —H —H, —H —H, —H 139 Cu

—Cl —H, —H —H, —H —H, —H —H, —H 140 Cu

—H —H, —H —H, —H —H, —H —H, —H 141 Cu

—H —H, —H —H, —H —H, —H —H, —H 142 Cu

—H —H, —H —H, —H —H, —H —H, —H 143 Cu

—H —H, —H —H, —H —H, —H —H, —H 144 Cu

—H —H, —H —H, —H —H, —H —H, —H 145 Cu —SO₂CH₂CH₂OCH₂CH₂OCH₂CH₂SO₃Li —H —H, —H —H, —H —H, —H —H, —H

In the following Tables, each introduction site of substituents (X_(p1)) and (X_(p2)) is in an irregular order within the β-position substitution type.

M—Pc(Xp₁)_(m)(Xp₂)_(n) No. M Xp₁ m 146 Cu

3 147 Cu —SO₂—NH—CH₂—CH₂SO₃Li 3 148 Cu

3 149 Cu

2 150 Cu —SO₂—NH—CH₂—CH₂—SO₂—NH—CH₂CH₂—COONa 3 151 Cu

3 152 Cu

2.5 153 Cu

2 154 Cu SO₂—CH₂—CH₂—CH₂—SO₃Li 3 155 Cu SO₂—CH₂—CH₂—CH₂—COOK 2 156 Cu SO₂—CH₂—CH₂—CH₂—SO₃Li 3 157 Cu —SO₂—CH₂—CH₂—O—CH₂—CH₂—SO₃Li 2 158 Cu

3 159 Cu —SO₂NHCH₂CH₂—SO₂Li 3 160 Cu —SO₂—CH₂—CH₂—O—CH₂— CH₂—O—CH₂—CH₂—SO₃Na 3 161 Cu —SO₂CH₂CH₂CH₂SO₃Li 3 162 Cu —SO₂CH₂CH₂CH₂SO₃Li 2 163 Cu —SO₂CH₂CH₂CH₂SO₃K 3 164 Cu —SO₂CH₂CH₂CH₂SO₃Li 2 165 Cu —CO—NH—CH₂—CH₂—SO₃K 3 166 Cu —CO—NH—CH₂—CH₂—SO₂—NH—CH₂—CH₂—COONa 3 167 Cu

2.5 168 Cu

2 169 Cu —CO₂—CH₂—CH₂—CH₂—SO₃Li 3 170 Cu —CO₂—CH₂—CH₂—CH₂COOK 2 171 Cu —CO₂—CH₂—CH₂—O—CH₂— CH₂—O—CH₂—CH₂—SO₃Na 3 172 Cu —SOCH₂CH₂OCH₂CH₂O—CH₂CH₂SO₃K 2 173 Cu

2 174 Cu

3 175 Cu —SO₂(CH₂)₃SO₂NH(CH₂)₃N(CH₂CH₂OH)₂ 2 176 Cu

3 177 Cu —SO₂—CH₂—CH₂—O—CH₂—CH₂—O—CH₃ 2 178 Cu —CO₂—CH₂—CH₂—O—CH₂— CH₂—O—CH₂—CH₂—OH 3 179 Cu

2 180 Cu

3 181 Cu

3 182 Cu

2.5 183 Cu

2 184 Cu

3 185 Cu

3 186 Cu

3 187 Cu —SO₂—CH₂—CH₂—CH₂—SO₂—NH—CHCH₃)₂ 3 188 Cu

3 189 Cu —CO—NH—CH₂—CH₂—SO₂—NH—CHCH₃)₂ 3 190 Cu

3 No. Xp₂ n 146

1 147

1 148 —SO₂NH—CH₂—CH₂—CH₂—SO₂—NH— CH₂—CH₂—O—CH₂—CH₂—OH 1 149 —SO₂—NH—CH₂—CH₂—CH₂—CO—NCH₂—CH₂—OH)₂ 2 150

1 151 —SO₂NH—CH₂—CH₂—O—CH₂—CH₂—OH 1 152 —SO₂—CH₂—CH₂—O—CH₂—CH₂—OH 1.5 153 —SO₂—CH₂—CH₂—CH₂—CO—NCH₂—CH₂—OH)₂ 2 154

1 155

2 156

1 157

2 158

1 159

1 160

1 161

1 162 —SO₂CH₂CH₂CH₂OCH₂CH₂OCH₂CH₂OH 2 163

1 164 —SO₂CH₂CH₂CH₂SO₂N(CH₂CH₂OH)₂ 2 165 —CO—NH—CH₂—CH₂—O—CH₂—CH₂—OH 2 166

1 167 —CO—NH—CH₂—CH₂—CH₂—CO—NCH₂—CH₂—OH)₂ 1.5 168 —CO—CH₂—CH₂—CH₂—CO—NCH₂—CH₂—OH)₂ 2 169

1 170

2 171

1 172

2 173

2 174

1 175

2 176

1 177

1 178

1 179

2 180 —SO₂NH—CH₂—CH₂—SO₂NH—CH₂— CH₂—O—CH₂—CH₂—OH 1 181 —SO₂—CH₂—CH₂—CH₂—SO₂—NH—CHCH₃)₂ 1 182

1.5 183 —SO₂—CH₂—CH₂—CH₂— SO₂—NH—(CH₂)₃—CH₂—O—CH₂CH₂—OH 2 184 —SO₂—CH₂—CH₂—O—CH₂—CH₂—O—CH₃ 1 185 —SO₂—CH₂—CH₂—O—CH₂—CH₂—O— CH₂—CH₂—O—CH₃ 1 186 —SO₂—CH₂—CH₂—O—CH₂—CH₂—O— CH₂—CH₂—OH 1 187

1 188 —CO₂—CH₂—CH₂—O—CH₂—CH₂—O—CH₃ 1 189

1 190 —CO—NH—CH₂—CH₂—O—CH₂—CH₂—O—CH₃ 1

The structural formula of the phthalocyanine compound represented by M-Pc(X_(p1))_(m)(X_(p2))_(n) in Tables 21 to 24 is shown below:

(wherein each X_(p1) is independently X_(p1) or X_(p2)).

The phthalocyanine dye represented by formula (I) can be synthesized according to the patent publications described above. Furthermore, the phthalocyanine dye represented by formula (II) can be synthesized by the methods described in JP-A-2001-226275, JP-A-2001-96610, JP-A-2001-47013 and JP-A-2001-193638, in addition to the synthesis method described above. The starting material, dye intermediate and synthesis route are not limited to those described in these patent publications.

The ink for use in the ink set of the present invention is described below except for the above-described coloring agent such as dye and pigment.

The surfactant which can be contained in the ink for use in the ink set of the present invention is described below.

In the present invention, a surfactant is incorporated into the ink to control the liquid properties of ink, whereby excellent effects can be provided, such as improvement of ejection stability of the ink, elevation of water resistance of the image and prevention of bleeding of the printed ink.

Examples of the surfactant include anionic surfactants such as sodium dodecylsulfate, sodium dodecyloxysulfonate and sodium alkylbenzenesulfonate, cationic surfactants such as cetyl pyridinium chloride, trimethylcetylammonium chloride and tetrabutylammonium chloride, and nonionic surfactants such as polyoxyethylene nonylphenyl ether, polyoxyethylene naphthyl ether and polyoxyethylene octylphenyl ether. Among these, nonionic surfactants are preferred.

The surfactant content is from 0.001 to 15 wt %, preferably from 0.005 to 10 wt %, more preferably from 0.01 to 5 wt %, based on the ink.

The ink for use in the ink set of the present invention can be prepared by dissolving and/or dispersing the above-described dye and surfactant in an aqueous medium. The term “aqueous medium” as used in the present invention means water or a mixture of water and a slight amount of water-miscible organic solvent, where additives such as wetting agent, stabilizer and antiseptic are added, if desired.

In preparing the ink solution of the present invention, in the case of a water-soluble ink, the dye is preferably first dissolved in water and thereafter, various solvents and additives are added, dissolved and mixed to provide a uniform ink solution.

For dissolving the dye and the like, various methods such as stirring, ultrasonic irradiation and shaking can be used. Among these, stirring is preferred. In performing the stirring, various systems known in the art can be used, such as flow stirring and stirring utilizing the shearing force by means of a reversal agitator or a dissolver. Also, a stirring method utilizing the shearing force with the bottom surface of a container, such as magnetic stirrer, can be advantageously used.

Examples of the water-miscible organic solvent which can be used in the present invention include alcohols (e.g., methanol, ethanol, propanol, isopropanol, butanol, isobutanol, sec-butanol, tert-butanol, pentanol, hexanol, cyclohexanol, benzyl alcohol), polyhydric alcohols (e.g., ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, thiodiglycol), glycol derivatives (e.g., ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol diacetate, ethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, ethylene glycol monophenyl ether), amines (e.g., ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenetriamine, triethylenetetramine, polyethyleneimine, tetramethylpropylenediamine) and other polar solvents (e.g., formamide, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, sulfolane, 2-pyrrolidone, N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, acetonitrile, acetone). These water-miscible organic solvents can be used in combination of two or more thereof.

In the case where the above-described dye is an oil-soluble dye, the ink solution can be prepared by dissolving the oil-soluble dye in a high boiling point organic solvent and emulsification-dispersing it in an aqueous medium.

The high boiling point organic solvent for use in the present invention has a boiling point of 150° C. or more, preferably 170° C. or more.

Examples thereof include phthalic acid esters (e.g., dibutyl phthalate, dioctyl phthalate, dicyclohexyl phthalate, di-2-ethylhexyl phthalate, decyl phthalate, bis(2,4-di-tert-amylphenyl) isophthalate, bis(1,1-diethylpropyl) phthalate), esters of phosphoric acid or phosphone (e.g., diphenyl phosphate, triphenyl phosphate, tricresyl phosphate, 2-ethylhexyldiphenyl phosphate, dioctylbutyl phosphate, tricyclohexyl phosphate, tri-2-ethylhexyl phosphate, tridodecyl phosphate, di-2-ethylhexylphenyl phosphate), benzoic acid esters (e.g., 2-ethylhexyl benzoate, 2,4-dichlorobenzoate, dodecyl benzoate, 2-ethylhexyl-p-hydroxybenzoate), amides (e.g., N,N-diethyldodecanamide, N,N-diethyllaurylamide), alcohols or phenols (e.g., isostearyl alcohol, 2,4-di-tert-amylphenol), aliphatic esters (e.g., dibutoxyethyl succinate, di-2-ethylhexyl succinate, 2-hexyldecyl tetradecanoate, tributyl citrate, diethyl azelate, isostearyl lactate, trioctyl citrate), aniline derivatives (e.g., N,N-dibutyl-2-butoxy-5-tert-octylaniline), chlorinated paraffins (e.g., paraffins having a chlorine content of 10 to 80%), trimesic acid esters (e.g., tributyl trimesate), dodecylbenzene, diisopropylnaphthalene, phenols (e.g., 2,4-di-tert-amylphenyl, 4-dodecyloxyphenol, 4-dodecyloxycarbonylphenol, 4-(4-dodecyloxyphenylsulfonyl)phenol), carboxylic acids (e.g., 2-(2,4-di-tert-amylphenoxy)butyric acid, 2-ethoxyoctanedecanoic acid), alkylphosphoric acids (e.g., di-(2-ethylhexyl)phosphoric acid, diphenylphosphoric acid). The high boiling point organic solvent can be used in an amount of, in terms of the mass ratio to the oil-soluble dye, from 0.01 to 3 times, preferably from 0.01 to 1.0 times. When a high boiling solvent is present, the dye and other nonvolatile components are less precipitated at the dispersion in the ink, as a result, the stability of ink is improved and good ejection stability is attained.

These high boiling point organic solvents may be used individually or as a mixture of several kinds [for example, tricresyl phosphate and dibutyl phthalate, trioctyl phosphate and di(2-ethylhexyl) sebacate, or dibutyl phthalate and poly(N-tert-butylacrylamide)].

Examples of the high boiling point organic solvent for use in the present invention, other than the above-described compounds, and/or the synthesis method of the high boiling organic solvent are described, for example, in U.S. Pat. Nos. 2,322,027, 2,533,514, 2,772,163, 2,835,579, 3,594,171, 3,676,137, 3,689,271, 3,700,454, 3,748,141, 3,764,336, 3,765,897, 3,912,515, 3,936,303, 4,004,928, 4,080,209, 4,127,413, 4,193,802, 4,207,393, 4,220,711, 4,239,851, 4,278,757, 4,353,979, 4,363,873, 4,430,421, 4,430,422, 4,464,464, 4,483,918, 4,540,657, 4,684,606, 4,728,599, 4,745,049, 4,935,321 and 5,013,639, EP-A-276319, EP-A-286253, EP-A-289820, EP-A-309158, EP-A-309159, EP-A-309160, EP-A-509311, EP-A-510576, East German Patents 147,009, 157,147, 159,573 and 225,240A, British Patent 2091124A, JP-A-48-47335, JP-A-50-26530, JP-A-51-25133, JP-A-51-26036, JP-A-51-27921, JP-A-51-27922, JP-A-51-149028, JP-A-52-46816, JP-A-53-1520, JP-A-53-1521, JP-A-53-15127, JP-A-53-146622, JP-A-54-91325, JP-A-54-106228, JP-A-54-118246, JP-A-55-59464, JP-A-56-64333, JP-A-56-81836, JP-A-59-204041, JP-A-61-84641, JP-A-62-118345, JP-A-62-247364, JP-A-63-167357, JP-A-63-214744, JP-A-63-301941, JP-A-64-9452, JP-A-64-9454, JP-A-64-68745, JP-A-1-101543, JP-A-1-102454, JP-A-2-792, JP-A-2-4239, JP-A-2-43541, JP-A-4-29237, JP-A-4-30165, JP-A-4-232946 and JP-A-4-346338.

The high boiling point organic solvent is used in an amount of, in terms of the mass ratio to the oil-soluble dye, from 0.01 to 3.0 times, preferably from 0.01 to 1.0 times.

In the present invention, the oil-soluble dye or high boiling point organic solvent is used by emulsification-dispersing it in an aqueous medium. Depending on the case, a low boiling point organic solvent may be also used at the emulsification-dispersion in view of emulsifiability. The low boiling point organic solvent is an organic solvent having a boiling point of about 30° C. to 150° C. at atmospheric pressure. Preferred examples thereof include esters (e.g., ethyl acetate, butyl acetate, ethyl propionate, β-ethoxyethyl acetate, methylcellosolve acetate), alcohols (e.g., isopropyl alcohol, n-butyl alcohol, secondary butyl alcohol), ketones (e.g., methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone), amides (e.g., dimethylformamide, N-methylpyrrolidone) and ethers (e.g., tetrahydrofurane, dioxane), however, the present invention is not limited thereto.

In the emulsification dispersion, an oil phase obtained by dissolving the dye in a high boiling organic solvent or depending on the case, in a mixed solvent of a high boiling organic solvent and a low boiling organic solvent is dispersed in an aqueous phase mainly comprising water to form fine oil droplets of the oil phase. At this time, in either one or both of the aqueous phase and the oil phase, additives described later, such as surfactant, wetting agent, dye stabilizer, emulsification stabilizer, antiseptic and fungicide, can be added, if desired.

In the general emulsification method, an oil phase is added to an aqueous phase, however, a so-called phase inversion emulsification method of adding dropwise an aqueous phase in an oil phase can also be preferably used. The above-described emulsification method can be applied also when the azo dye used in the present invention is water-soluble and the additives are oil-soluble.

In performing the emulsification dispersion, various surfactants can be used. Preferred examples thereof include anionic surfactants such as fatty acid salt, alkylsulfuric ester salt, alkylbenzenesulfonate, alkylnaphthalenesulfonate, dialkylsulfosuccinate, alkylphosphoric ester salt, naphthalenesulfonic acid formalin condensate and polyoxyethylene alkylsulfuric ester salt, and nonionic surfactants such as polyoxyethylene alkyl ether, polyoxyethylene alkylallyl ether, polyoxyethylene fatty acid ester, sorbitan fatty acid ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkylamine, glycerin fatty acid ester and oxyethylene oxypropylene block copolymer. Also, SURFYNOLS (produced by Air Products & Chemicals), which is an acetylene-base polyoxyethylene oxide surfactant, is preferably used. In addition, amine oxide-type amphoteric surfactants such as N,N-dimethyl-N-alkylamine oxide are preferred. Furthermore, surfactants described in JP-A-59-157636 (pages (37) to (38)) and Research Disclosure, No. 308119 (1989) can be used.

For the purpose of stabilizing the dispersion immediately after the emulsification, a water-soluble polymer may also be added in combination with the surfactant. As the water-soluble polymer, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene oxide, polyacrylic acid, polyacrylamide, and their copolymer are preferably used. In addition, natural water-soluble polymers such as polysaccharides, casein and gelatin are also preferably used. Furthermore, for the stabilization of the dye dispersion, a polymer which does not substantially dissolve in an aqueous medium, such as polyvinyl, polyurethane, polyester, polyamide, polyurea and polycarbonate obtained by the polymerization of acrylic acid esters, methacrylic acid esters, vinyl esters, acrylamides, methacrylamides, olefins, styrenes, vinyl ethers or acrylonitriles, can also be used in combination. This polymer preferably contains —SO₃ ⁻ or —COO⁻. In the case of using this polymer which does not substantially dissolve in an aqueous medium, the polymer is preferably used in an amount of 20 wt % or less, more preferably 10 wt % or less, based on the high boiling point organic solvent.

In preparing an aqueous ink by dispersing an oil-soluble dye or a high boiling point organic solvent using the emulsification dispersion, control of the particle size is important. In order to elevate the color purity or density of an image formed by the inkjet recording, it is an essential matter to reduce the average particle size. The average particle size is, in terms of the volume average particle size, preferably 1 μm or less, more preferably from 5 to 100 nm.

The volume average particle size and particle size distribution of the dispersed particles can be easily measured by a known method such as static light scattering method, dynamic light scattering method, centrifugal precipitation method and the method described in Jikken Kagaku Koza (Lecture of Experimental Chemistry), 4th ed., pp. 417-418. For example, the ink solution is diluted with distilled water such that the particle concentration in the ink becomes from 0.1 to 1 wt %, then, the particle size can be easily measured by a commercially available volume average particle size measuring apparatus (for example, Microtrac UPA, manufactured by Nikkiso K. K.). The dynamic light scattering method utilizing the laser Doppler effect is particularly preferred because even a small particle size can be measured.

The volume average particle size is an average particle size weighted with the particle volume and is obtained by multiplying the diameter of individual particles in the gathering of particles with the volume of the particle and dividing the sum total of the obtained values by the total volume of the particles. The volume average particle size is described in Soichi Muroi, Kobunshi Latex no Kagaku (Chemistry of Polymer Latex), page 119, Kobunshi Kanko Kai.

Also, it is revealed that the presence of coarse particles greatly affects the printing performance. More specifically, the coarse particle clogs the nozzle of head or even if the nozzle is not clogged, forms a soil to bring about failure or slippage in the ejection of ink and the printing performance is seriously affected by this. In order to prevent these troubles, it is important to reduce the number of particles having a particle size of 5 μm or more to 10 or less and the number of particles having a particle size of 1 μm or more to 100 or less, in 1 μl of ink prepared.

For removing these coarse particles, a known method such as centrifugal separation or microfiltration can be used. This separation step may be performed immediately after the emulsification dispersion or may be performed immediately before the filling in an ink cartridge after various additives such as wetting agent and surfactant are added to the emulsified dispersion.

A mechanically emulsifying apparatus is effective for reducing the average particle size and eliminating coarse particles.

As the emulsifying apparatus, known apparatuses such as simple stirrer, impeller stirring system, in-line stirring system, mill system (e.g., colloid mill) and ultrasonic system can be used, however, a high-pressure homogenizer is particularly preferred.

The mechanism of the high-pressure homogenizer is described in detail in U.S. Pat. No. 4,533,254 and JP-A-6-47264. Examples of the commercially available apparatus include Gaulin Homogenizer (manufactured by A. P. V Gaulin Inc.), Microfluidizer (manufactured by Microfluidex Inc.) and Altimizer (produced by Sugino Machine).

The high-pressure homogenizer with a mechanism of pulverizing particles in an ultrahigh pressure jet stream recently described in U.S. Pat. No. 5,720,551 is particularly effective for the emulsification dispersion of the present invention. Examples of the emulsifying apparatus using this ultrahigh pressure jet stream include DeBEE2000 (manufactured by BEE International Ltd.).

In performing the emulsification by a high-pressure emulsification dispersing apparatus, the pressure is 50 MPa or more, preferably 60 MPa or more, more preferably 180 MPa or more.

A method of using two or more emulsifying apparatuses, for example, emulsification in a stirring emulsifier and then passing through a high-pressure homogenizer, is particularly preferred. Also, a method of once emulsification dispersing the dye solution by such an emulsifying apparatus and after adding additives such as wetting agent and surfactant, again passing the dispersion through a high-pressure homogenizer during the time of filling the ink into a cartridge is preferred.

In the case of containing a low boiling point organic solvent in addition to a high boiling point organic solvent, the low boiling point solvent is preferably removed in view of stability of the emulsified product, safety and hygiene. For removing the low boiling point solvent, various known methods can be used according to the kind of solvent. Examples of the method include evaporation, vacuum evaporation and ultrafiltration. This removal of the low boiling point organic solvent is preferably performed as soon as possible immediately after the emulsification.

The preparation method of the ink for inkjet use is described in detail in JP-A-5-148436, JP-A-5-295312, JP-A-7-97541, JP-A-7-82515 and JP-A-7-118584 and those described in these patent publications can be utilized also in the preparation of ink used in the ink set for inkjet use of the present invention.

In the ink for inkjet recording, which is used in the ink set of present invention, additives such as drying inhibitor for preventing clogging due to drying of ink at the ejection port, permeation accelerator for attaining more successful permeation of ink into paper, ultraviolet absorbent, antioxidant, viscosity adjusting agent, surface tension adjusting agent, dispersant, dispersion stabilizer, fungicide, rust inhibitor, pH adjusting agent, defoaming agent and chelating agent can be appropriately selected and used in an appropriate amount.

The drying inhibitor is preferably a water-soluble organic solvent having a vapor pressure lower than water. Specific examples thereof include polyhydric alcohols represented by ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycol, thiodiglycol, dithiodiglycol, 2-methyl-1,3-propanediol, 1,2,6-hexanetriol, acetylene glycol derivative, glycerin and trimethylolpropane; lower alkyl ethers of polyhydric alcohol, such as ethylene glycol monomethyl(or ethyl) ether, diethylene glycol monomethyl(or ethyl) ether and triethylene glycol monoethyl(or butyl) ether; heterocyclic rings such as 2-pyrrolidone, N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone and N-ethylmorpholine; sulfur-containing compounds such as sulfolane, dimethylsulfoxide and 3-sulfolene; polyfunctional compounds such as diacetone alcohol and diethanolamine; and urea derivatives. Among these, polyhydric alcohols such as glycerin and diethylene glycol are preferred. These drying inhibitors may be used individually or in combination of two or more thereof. The drying inhibitor is preferably contained in the ink in an amount of 10 to 50 wt %.

Examples of the permeation accelerator include alcohols such as ethanol, isopropanol, butanol, di(tri)ethylene glycol monobutyl ether and 1,2-hexanediol, sodium laurylsulfate, sodium oleate and nonionic surfactants. A sufficiently high effect can be obtained by adding from 10 to 30 wt % of the permeation accelerator to the ink. The permeation accelerator is preferably used in an amount of causing no bleeding of printed letter or no print through.

In the ink for the ink set of the present invention, an ultraviolet absorbent is used for improving the storage stability of image and examples of the ultraviolet absorbent which can be used include benzotriazole-base compounds described in JP-A-58-185677, JP-A-61-190537, JP-A-2-782, JP-A-5-197075 and JP-A-9-34057, benzophenone-base compounds described in JP-A-46-2784, JP-A-5-194483 and U.S. Pat. No. 3,214,463, cinnamic acid-base compounds described in JP-B-48-30492 (the term “JP-B” as used herein means an “examined Japanese patent publication”), JP-B-56-21141 and JP-A-10-88106, triazine-base compounds described in JP-A-4-298503, JP-A-8-53427, JP-A-8-239368, JP-A-10-182621 and JP-T-8-501291 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”), compounds described in Research Disclosure No. 24239, and compounds of absorbing ultraviolet light and emitting fluorescent light, so-called fluorescent brightening agents, represented by stilbene-base compounds and benzoxazole-base compounds.

As the antioxidant which is used for improving the storage stability of image, various organic discoloration inhibitors and metal complex-base discoloration inhibitors can be used. Examples of the organic discoloration inhibitor include hydroquinones, alkoxyphenols, dialkoxyphenols, phenols, anilines, amines, indanes, chromans, alkoxyanilines and heterocyclic rings. Examples of the metal complex include nickel complex and zinc complex. More specifically, compounds described in patents cited in Research Disclosure, Nos. 17643 (Items VII-I to VII-J), 15162, 18716 (page 650, left column), 36544 (page 527), 307105 (page 872) and 15162, and compounds included in formulae of representative compounds and in exemplary compounds described in JP-A-62-215272 (pages 127 to 137) can be used.

Examples of the fungicide for use in the ink include sodium dehydroacetate, sodium benzoate, sodium pyridine-thione-1-oxide, ethyl p-hydroxybenzoate, 1,2-benziso-thiazolin-3-one and salts thereof. The fungicide is preferably used in the ink in an amount of 0.02 to 5.00 wt %.

These are described in detail in Bokin Bobai Zai Jiten (Dictionary of Microbicide and Fungicide), compiled by Nippon Bokin Bobai Gakkai Jiten Henshu Iinkai.

Examples of the rust inhibitor include acidic sulfite, sodium thiosulfate, ammon thioglycolate, diisopropylammonium nitrite, pentaerythritol tetranitrate, dicyclohexylammonium nitrite and benzotriazole. The rust inhibitor is preferably used in the ink in an amount of 0.02 to 5.00 wt %.

The pH adjusting agent added to the ink for the ink set of the present invention is suitably used for adjusting the pH and imparting dispersion stability. The pH of the ink is preferably adjusted to 4 to 11 at 25° C. If the pH is less than 4, the solubility of dye decreases to readily cause clogging of a nozzle, whereas if it exceeds 11, the water resistance is liable to deteriorate. Examples of the pH adjusting agent include, as basic one, organic bases and inorganic alkalis, and as acidic one, organic acids and inorganic acids.

Examples of the basic compound which can be used include inorganic compounds such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogencarbonate, potassium hydrogencarbonate, sodium acetate, potassium acetate, sodium phosphate and sodium monohydrogenphosphate; and organic bases such as aqueous ammonia, methylamine, ethylamine, diethylamine, triethylamine, ethanolamine, diethanolamine, triethanolamine, ethylenediamine, piperidine, diazabicyclooctane, diazabicycloundecene, pyridine, quinoline, picoline, lutidine and collidine.

Examples of the acidic compound which can be used include inorganic compounds such as hydrochloric acid, sulfuric acid, phosphoric acid, sodium hydrogensulfate, potassium hydrogensulfate, potassium dihydrogenphosphate and sodium dihydrogenphosphate; and organic compounds such as acetic acid, tartaric acid, benzoic acid, trifluoroacetic acid, methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, saccharinic acid, phthalic acid, picolinic acid and quinolinic acid.

Each constituent ink of the ink set preferably has a conductivity of 0.01 to 10 S/m. The conductivity is more preferably from 0.05 to 5 S/m.

The conductivity can be measured by an electrode method using a commercially available saturated potassium chloride.

The conductivity can be controlled mainly by the ion concentration in an aqueous solution. In the case where the salt concentration is high, desalting can be performed using ultrafiltration membrane or the like. Also, in the case of controlling the conductivity by adding a salt or the like, the conductivity can be controlled by adding various organic or inorganic salts.

Examples of the inorganic salt which can be used include inorganic compounds such as potassium halide, sodium halide, sodium sulfate, potassium sulfate, sodium hydrogensulfate, potassium hydrogensulfate, sodium nitrate, potassium nitrate, sodium hydrogencarbonate, potassium hydrogencarbonate, sodium phosphate, sodium monohydrogenphosphate, boric acid, potassium dihydrogenphosphate and sodium dihydrogenphosphate. Also, organic compounds such as sodium acetate, potassium acetate, potassium tartrate, sodium tartrate, sodium benzoate, potassium benzoate, sodium p-toluenesulfonate, potassium saccharinate, potassium phthalate and sodium picolinate can be used.

The conductivity can also be controlled by selecting the components of the aqueous medium which is described later.

The ink of the present invention preferably has a viscosity at 25° C. of 1 to 20 mPa·s, more preferably from 2 to 15 mPa·s, still more preferably from 2 to 10 mPa·s. If the viscosity exceeds 30 mPa·s, the fixing rate of the recorded image decreases and the ejection performance also decreases, whereas if it is less than 1 mPa·s, the recorded image is blurred and therefore, decreased in the grade.

The viscosity can be freely adjusted by the amount of the ink solvent added. Examples of the ink solvent include glycerin, diethylene glycol, triethanolamine, 2-pyrrolidone, diethylene glycol monobutyl ether and triethylene glycol monobutyl ether.

A viscosity adjusting agent may also be used. Examples of the viscosity adjusting agent include celluloses, water-soluble polymers such as polyvinyl alcohol, and nonionic surfactants. These are described in detail in Nendo Chosei Gijutsu (Viscosity Adjusting Technology), Chap. 9, Gijutsu Joho Kyokai (1999), and Inkjet Printer Yo Chemicals (98 Zoho)—Zairyo no Kaihatsu Doko•Tenbo Chosa—(Chemicals for Inkjet Printer (Enlarged Edition of 98)—Survey on Tendency•Prospect of Development of Materials—), pp. 162-174, CMC (1997).

The method for measuring the viscosity of liquid is described in detail in JIS Z8803 but the viscosity can be easily measured by a commercially available viscometer and examples of the rotational viscometer include B-type viscometer and E-type viscometer, manufactured by Tokyo Keiki Co. In the present invention, the viscosity is measured at 25° C. using a vibrating visco-meter Model VM-100A-L manufactured by Yamaichi Denki. The unit of viscosity is pascal second (Pa·s) but usually, milli-pascal second (mPa·s) is used.

Whichever dynamic surface tension or static surface tension is used, the surface tension of the ink for use in the present invention is preferably from 20 to 50 mN/m, more preferably from 20 to 40 mN/m, at 25° C. If the surface tension exceeds 50 mN/m, ejection stability and printing quality are seriously deteriorated, for example, bleeding at color mixing or feathering is caused, whereas if the surface tension of ink is less than 20 mN/m, printing failure may occur due to attachment of ink to the hard surface at the ejection.

For the purpose of adjusting the surface tension, a cationic, anionic or nonionic surfactant of various types can be added. The surfactant is preferably used in the range from 0.01 to 20 wt %, more preferably from 0.1 to 10 wt %, based on the ink for inkjet use. The surfactants can be used in combination of two or more thereof.

As the method for measuring the static surface tension, a capillary elevation method, a dropping method, a suspended ring method and the like are known. In the present invention, a vertical plate method is used as the method for measuring the static surface tension.

When a glass or platinum thin plate is vertically hung while dipping a part of the plate in a liquid, a surface tension of the liquid acts downward along the length of contact between the liquid and the plate. This force is balanced with an upward force and thereby, the surface tension can be measured.

As the method for measuring the dynamic surface tension, a vibrating jet method, a meniscus dropping method, a maximum bubble pressure method and the like are known as described, for example, in Shin Jikken Kagaku Koza, Kaimen to Colloid (New Lecture of Experimental Chemistry, Interface and Colloid), Vol. 18, pp. 69-90, Maruzen (1977). Furthermore, a liquid film rupturing method described in JP-A-3-2064 is known. In the present invention, a differential bubble pressure method is used as the method for measuring the dynamic surface tension. The principle and method of the measurement are described below.

When bubbles are produced in a solution rendered uniform by stirring, a gas-liquid interface is newly produced and surfactant molecules in the solution gather to the water surface at a constant speed. When the bubble rate (bubble production rate) is changed, as the production rate decreases, a larger number of surfactant molecules gather to the bubble surface. Therefore, the maximum bubble pressure immediately before the bubble bursts becomes small and the maximum bubble pressure (surface tension) for the bubble rate can be detected. The dynamic surface tension is preferably measured by a method of producing bubbles in a solution using large and small two probes, measuring the differential pressure in the maximum bubble pressure state between two probes, and calculating the dynamic surface tension.

In view of ejection stability of ink, printed image quality, various fastnesses of image or reduction in bleeding of image after printing or in stickiness on the printed surface, the nonvolatile component in the ink of the present invention is preferably from 10 to 70 wt % of the entire amount of the ink. In view of ejection stability of ink and reduction in bleeding of image after printing, the nonvolatile component is more preferably from 20 to 60 wt %.

The nonvolatile component as used herein means a liquid or solid component having a boiling point of 150° C. or more at 1 atm or a high molecular amount component. The nonvolatile component in the ink for inkjet use includes a dye and a high boiling point solvent and also includes a polymer latex, a surfactant, a dye stabilizer, a fungicide and a buffering agent which are added, if desired. Many of these nonvolatile components except for the dye stabilizer reduce the dispersion stability and even after printing, remain on the inkjet image-receiving paper to inhibit the aggregation and in turn stabilization of dye on the image-receiving paper and worsen various fastnesses of the image area or bleeding of the image under high humidity condition.

In the present invention, a high molecular amount compound may be contained. The high molecular amount compound as used herein means all polymer compounds having a number average molecular weight of 5,000 or more contained in the ink. Examples of the polymer compound include a water-soluble polymer compound which substantially dissolves in an aqueous medium, a water-dispersible polymer compound such as polymer latex and polymer emulsion, and an alcohol-soluble polymer compound which dissolves in a polyhydric alcohol used as an auxiliary solvent, however, the high molecular weight compound as used in the present invention includes any compound which substantially dissolves or disperses uniformly in the ink solution.

Specific examples of the water-soluble polymer compound include water-soluble polymers such as polyvinyl alcohol, silanol-modified polyvinyl alcohol, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinylpyrrolidone, polyalkylene oxide (e.g., polyethylene oxide, polypropylene oxide) and polyalkylene oxide derivatives; natural water-soluble polymers such as polysaccharides, starch, cationized starch, casein and gelatin; aqueous acrylic resins such as polyacrylic acid, polyacrylamide and copolymers thereof; aqueous alkyd resin; and water-soluble polymer compounds having a —SO₃ ⁻ or —COO⁻ group within the molecule and substantially dissolving in an aqueous medium.

Specific examples of the polymer latex include a styrene-butadiene latex, a styrene-acryl latex and a polyurethane latex, and specific examples of the polymer emulsion include an acryl emulsion.

These water-soluble polymer compounds can be used individually or in combination of two or more thereof.

As described above, the water-soluble polymer compound is used as the viscosity adjusting agent so as to adjust the viscosity of ink to a viscosity region of giving good ejection property, however, if the amount of the water-soluble polymer compound added is large, the viscosity of ink increases to reduce the ejection stability of ink solution and after aging of ink, the nozzle is readily clogged by the precipitate.

The amount added of the polymer compound as the viscosity adjusting agent varies depending on the molecular weight of the compound added (as the molecular weight is higher, the amount added can be smaller), but the amount added is from 0 to 5 wt %, preferably from 0 to 3 wt %, more preferably from 0 to 1 wt %, based on the entire amount of ink.

In the present invention, apart from the above-described surfactant, a nonionic, cationic or anionic surfactant is used as the surface tension adjusting agent. Examples of the anionic surfactant include a fatty acid salt, an alkylsulfuric ester salt, an alkylbenzenesulfonate, an alkylnaphthalenesulfonate, a dialkylsulfosuccinate, an alkylphosphoric ester salt, a naphthalenesulfonic acid formalin condensate and a polyoxyethylenealkylsulfuric ester salt. Examples of the nonionic surfactant include a polyoxyethylene alkyl ether, a polyoxyethylene alkylallyl ether, a polyoxyethylene fatty acid ester, a sorbitan fatty acid ester, a polyoxyethylene sorbitan fatty acid ester, a polyoxyethylene alkylamine, a glycerin fatty acid ester and an oxyethylene oxypropylene block copolymer. Also, SURFYNOLS (produced by Air Products & Chemicals), which is an acetylene-base polyoxyethylene oxide surfactant, is preferably used. In addition, amine oxide-type amphoteric surfactants such as N,N-dimethyl-N-alkylamine oxide are preferred. Furthermore, surfactants described in JP-A-59-157636 (pages (37) to (38)) and Research Disclosure, No. 308119 (1989) can be used.

The surface tension of the ink of the present invention is, with or without use of such a surfactant, preferably from 20 to 60 mN/m, more preferably from 25 to 45 mN/m.

In the present invention, if desired, various cationic, anionic or nonionic surfactants described above may be used as a dispersant or a dispersion stabilizer, and a fluorine- or silicone-base compound or a chelating agent represented by EDTA may be used as a defoaming agent.

The recording paper and recording film used in the inkjet recording to which the present invention is applied are described below. The support which can be used for the recording paper or film is produced, for example, from a chemical pulp such as LBKP and NBKP, a mechanical pulp such as GP, PGW, RMP, TMP, CTMP, CMP and CGP, a waste paper pulp such as DIP, by mixing, if desired, conventionally known additives such as pigment, binder, sizing agent, fixing agent, cation agent and paper strength increasing agent, and then sheeting the mixture using various devices such as Fourdrinier paper machine and cylinder paper machine. Other than these supports, synthetic paper or plastic film may be used. The thickness of the support is preferably from 10 to 250 μm and the basis weight is preferably from 10 to 250 g/m².

An image-receiving layer and a backcoat layer may be provided on the support as it is to produce an image-receiving material for the ink of the present invention, or an image-receiving layer and a backcoat layer may be provided after providing a size press or anchor coat layer using starch, polyvinyl alcohol or the like to produce an image-receiving material. The support may also be subjected to a flattening treatment by a calendering device such as machine calender, TG calender and soft calender.

The support is preferably paper or plastic film of which both surfaces are laminated with polyolefin (for example, polyethylene, polystyrene, polybutene or a copolymer thereof) or polyethylene terephthalate. In the polyolefin, a white pigment (for example, titanium oxide or zinc oxide) or a tinting dye (for example, cobalt blue, ultramarine or neodymium oxide) is preferably added.

The image-receiving layer provided on the support contains a porous material and an aqueous binder. Also, the image-receiving layer preferably contains a pigment and the pigment is preferably a white pigment. Examples of the white pigment include inorganic white pigments such as calcium carbonate, kaolin, talc, clay, diatomaceous earth, synthetic amorphous silica, aluminum silicate, magnesium silicate, calcium silicate, aluminum hydroxide, alumina, lithopone, zeolite, barium sulfate, calcium sulfate, titanium dioxide, zinc sulfide and zinc carbonate, and organic pigments such as styrene-base pigment, acryl-base pigment, urea resin and melamine resin. Among these, porous inorganic white pigments are preferred, and synthetic amorphous silica and the like having a large pore area are more preferred. The synthetic amorphous silica may be either a silicic acid anhydride obtained by a dry production method (gas phase method) or a silicic acid hydrate obtained by a wet production method.

Specific examples of the recording paper containing the pigment in the image-receiving layer include those disclosed in JP-A-10-81064, JP-A-10-119423, JP-A-10-157277, JP-A-10-217601, JP-A-11-348409, JP-A-2001-138621, JP-A-2000-43401, JP-A-2000-211235, JP-A-2000-309157, JP-A-2001-96897, JP-A-2001-138627, JP-A-11-91242, JP-A-8-2087, JP-A-8-2090, JP-A-8-2091, JP-A-8-2093, JP-A-8-174992, JP-A-11-192777 and JP-A-2001-301314.

Examples of the aqueous binder contained in the image-receiving layer include water-soluble polymers such as polyvinyl alcohol, silanol-modified polyvinyl alcohol, starch, cationized starch, casein, gelatin, carboxymethyl cellulose, hydroxyethyl cellulose, polyvinylpyrrolidone, polyalkylene oxide and polyalkylene oxide derivatives, and water-dispersible polymers such as styrene butadiene latex and acryl emulsion. These aqueous binders can be used individually or in combination of two or more thereof. Among these, polyvinyl alcohol and silanol-modified polyvinyl alcohol are preferred in the present invention in view of adhesion to the pigment and peeling resistance of the ink-accepting layer.

The image-receiving layer may contain a mordant, a water-proofing agent, a light fastness enhancer, a gas resistance enhancer, a surfactant, a hardening agent and other additives in addition to the pigment and the aqueous binder.

The mordant added to the image-receiving layer is preferably immobilized and for this purpose, a polymer mordant is preferably used.

The polymer mordant is described in JP-A-48-28325, JP-A-54-74430, JP-A-54-124726, JP-A-55-22766, JP-A-55-142339, JP-A-60-23850, JP-A-60-23851, JP-A-60-23852, JP-A-60-23853, JP-A-60-57836, JP-A-60-60643, JP-A-60-118834, JP-A-60-122940, JP-A-60-122941, JP-A-60-122942, JP-A-60-235134, JP-A-1-161236 and U.S. Pat. Nos. 2,484,430, 2,548,564, 3,148,061, 3,309,690, 4,115,124, 4,124,386, 4,193,800, 4,273,853, 4,282,305 and 4,450,224. An image-receiving material containing the polymer mordant described in JP-A-1-161236 (pages 212 to 215) is particularly preferred. When the polymer mordant described in this patent publication is used, an image having excellent image quality can be obtained and at the same time, light fastness of the image is improved.

The water-proofing agent is effective for obtaining a water-resistant image. The water-proofing agent is preferably a cationic resin. Examples of the cationic resin include polyamidopolyamine epichlorohydrin, polyethyleneimine, polyaminesulfone, poly-dimethyldiallyl-ammonium chloride and cation polyacrylamide. The content of the cationic resin is preferably from 1 to 15 wt %, more preferably from 3 to 10 wt %, based on the entire solid content of the ink-accepting layer.

Examples of the light fastness enhancer and the gas resistance enhancer include phenol compounds, hindered phenol compounds, thioether compounds, thiourea compounds, thiocyanic acid compounds, amine compounds, hindered amine compounds, TEMPO compounds, hydrazine compounds, hydrazide compounds, amidine compounds, vinyl group-containing compounds, ester compounds, amide compounds, ether compounds, alcohol compounds, sulfinic acid compounds, saccharides, water-soluble reducing compounds, organic acids, inorganic acids, hydroxy group-containing organic acids, benzotriazole compounds, benzophenone compounds, triazine compounds, heterocyclic compounds, water-soluble metal salts, organic metal compounds and metal complexes.

Specific examples of these compounds include those described in JP-A-10-182621, JP-A-2001-260519, JP-A-2000-260519, JP-B-4-34953, JP-B-4-34513, JP-B-4-34512, JP-A-11-170686, JP-A-60-67190, JP-A-7-276808, JP-A-2000-94829, JP-T-8-512258 and JP-A-11-321090.

The surfactant functions as a coating aid, a releasability improver, a slipperiness improver or an antistatic agent. The surfactant is described in JP-A-62-173463 and JP-A-62-183457.

In place of the surfactant, an organic fluoro compound may be used. The organic fluoro compound is preferably hydrophobic. Examples of the organic fluoro compound include fluorine-containing surfactants, oily fluorine-base compounds (for example, fluorine oil) and solid fluorine compound resins (for example, ethylene tetrafluoride resin). The organic fluoro compound is described in JP-B-57-9053 (columns 8 to 17), JP-A-61-20994 and JP-A-62-135826.

As the hardening agent, for example, the materials described in JP-A-1-161236 (page 222), JP-A-9-263036, JP-A-10-119423 and JP-A-2001-310547 can be used.

Other additives added to the image-receiving layer include a pigment dispersant, a thickener, a defoaming agent, a dye, a fluorescent brightening agent, an antiseptic, a pH adjusting agent, a matting agent, a hardening agent and the like. The ink-accepting layer may be composed of either one layer or two layers.

In the recording paper or film, a backcoat layer may also be provided. Examples of the component which can be added to this layer include a white pigment, an aqueous binder and other components.

Examples of the white pigment contained in the backcoat layer include white inorganic pigments such as precipitated calcium carbonate, heavy calcium carbonate, kaolin, talc, calcium sulfate, barium sulfate, titanium dioxide, zinc oxide, zinc sulfide, zinc carbonate, satin white, aluminum silicate, diatomaceous earth, calcium silicate, magnesium silicate, synthetic amorphous silica, colloidal silica, colloidal alumina, pseudo-boehmite, aluminum hydroxide, alumina, lithopone, zeolite, hydrolyzed halloysite, magnesium carbonate and magnesium hydroxide, and organic pigments such as styrene-base plastic pigment, acryl-base plastic pigment, polyethylene, microcapsule, urea resin and melamine resin.

Examples of the aqueous binder contained in the backcoat layer include water-soluble polymers such as styrene/maleate copolymer, styrene/acrylate copolymer, polyvinyl alcohol, silanol-modified polyvinyl alcohol, starch, cationized starch, casein, gelatin, carboxymethyl cellulose, hydroxyethyl cellulose and polyvinylpyrrolidone, and water-dispersible polymers such as styrene butadiene latex and acryl emulsion. Other components contained in the backcoat layer include a defoaming agent, a foam inhibitor, a dye, a fluorescent brightening agent, an antiseptic, a water-proofing agent and the like.

In a constituent layer (including the back layer) of the inkjet recording paper or film, a polymer fine particle dispersion may be added. The polymer fine particle dispersion is used for the purpose of improving film properties, for example, stabilizing dimension and preventing curling, adhesion or film cracking. The polymer fine particle dispersion is described in JP-A-62-245258, JP-A-62-1316648 and JP-A-62-110066. When a polymer fine particle dispersion having a low glass transition temperature (40° C. or less) is added to a layer containing a mordant, the layer can be prevented from cracking or curling. The curling can be prevented also by adding a polymer fine particle dispersion having a high glass transition temperature to a back layer.

The ink set of the present invention is not limited on the inkjet recording system to which the ink set is applied, and is used in a known system, for example, an electric charge controlling system of jetting out the ink by utilizing the electrostatic induction force, a drop-on-demand system (pressure pulse system) utilizing an oscillation pressure of a piezo device, an acoustic inkjet system of converting electric signals into acoustic beams, irradiating the beams on the ink and jetting out the ink using the radiation pressure, or a thermal inkjet (bubble jet) system of heating the ink to form bubbles and utilizing the generated pressure.

The inkjet recording system includes a system of ejecting a large number of small-volume ink droplets of so-called photo ink having a low concentration, a system of using a plurality of inks substantially same in the color hue but different in the concentration and thereby improving the image quality, and a system using colorless transparent ink.

The ink for inkjet recording of the present invention can be used for uses other than the inkjet recording, such as a material for display image, an image-forming material for interior decoration and an image-forming material for outdoor decoration.

Examples of the material for display image include various materials such as poster, wall paper, ornamental goods (e.g., ornament, doll), handbill for commercial advertisement, wrapping paper, wrapping material, paper bag, vinyl bag, package material, billboard, image drawn on or attached to the side face of traffic (e.g., automobile, bus, electric car), and clothes with a logo. In the case of using the dye of the present invention as a material for forming a display image, the image includes, in addition to a strict image, all patterns by a dye, which can be acknowledged by a human, such as abstract design, letter and geometrical pattern.

Examples of the material for interior decoration include various materials such as wall paper, ornamental goods (e.g., ornament, doll), member of luminaire, member of furniture and design member of floor or ceiling. In the case of using the dye of the present invention as a material for forming an image, the image includes, in addition to a strict image, all patterns by a dye, which can be acknowledged by a human, such as abstract design, letter and geometrical pattern.

Examples of the material for outdoor decoration include various materials such as wall material, roofing material, billboard, gardening material, outdoor ornamental goods (e.g., ornament, doll), and member of outdoor luminaire. In the case of using the dye of the present invention as a material for forming an image, the image includes, in addition to a strict image, all patterns by a dye, which can be acknowledged by a human, such as abstract design, letter and geometrical pattern.

In these uses, examples of the medium where the pattern is formed include various materials such as paper, fiber, cloth (including non-woven fabric), plastic, metal and ceramic. Examples of the dyeing form include mordanting, printing, and fixing of a dyestuff in the form of a reactive dye having introduced thereinto a reactive group. Among these, preferred is dyeing by mordanting.

In the production of ink, sonic vibrations may be added, for example, in the step of dissolving additives such as dye.

The sonic vibration is added so as to prevent the ink from generation of bubbles due to a pressure applied in the recording head. More specifically, a sonic energy equal to or greater than the energy imposed in the recording head is previously applied in the process of producing the ink and thereby bubbles are eliminated.

The sonic vibration is usually an ultrasonic wave having a frequency of 20 kHz or more, preferably 40 kHz or more, more preferably 50 kHz or more. The energy added to liquid by the sonic vibration is usually 2×10⁷ J/m³ or more, preferably 5×10⁷ J/m³ or more, more preferably 1×10⁸ J/m³ or more. The time period where the sonic vibration is applied is usually on the order from 10 minutes to one hour.

No matter when the step of adding sonic vibrations is performed, the effect can be attained as long as it is performed after the dye is charged into a medium. The effect is also provided even by adding sonic vibrations after the finished ink is once stored. However, the sonic vibration is preferably added at the time of dissolving and/or dispersing the dye in a medium, because the effect of removing bubbles is large and the dissolution and/or dispersion of a dyestuff in a medium is accelerated by the sonic vibration.

That is, the step of adding at least sonic vibrations can be performed during or after the step of dissolving and/or dispersing the dye in a medium. In other words, the step of adding at least sonic vibrations can be optionally performed once or more after the preparation of ink until the ink is finished as a product.

In a practical embodiment, the step of dissolving and/or dispersing the dye in a medium preferably comprises a step of dissolving the dye in a partial medium out of the entire medium and a step of mixing the remaining medium. The sonic vibration is preferably added at least in either one of these steps, more preferably in the step of dissolving the dye in a partial medium out of the entire medium.

The step of mixing the remaining solvent may be a single step or a multiple step.

In the production of ink of the present invention, a degassing step under heating or reduced pressure is preferably used in combination, because the effect of removing bubbles in ink is enhanced. The degassing step under heating or reduced pressure is preferably performed simultaneously with or after the step of mixing the remaining medium.

Examples of the sonic vibration-generating means for use in the step of adding sonic vibrations include known devices such as ultrasonic disperser.

In the manufacture of the ink of the present invention, the step of removing dusts as the solid content by filtration, which is performed after the preparation of ink solution, is important. This operation is performed using a filtration filter and the filtration filter used here is a filter having an effective size of 1 μm or less, preferably from 0.05 to 0.3 μm, more preferably from 0.25 to 0.3 μm. For the construction material of the filter, various materials can be used, however, in the case of an ink using a water-soluble dye, a filter produced for aqueous solvents is preferably used. In particular, a filter made of a polymer material, which less generates wastes, is preferred. The filtration may be performed by feeding and passing the solution through a filter, and filtration under pressure or filtration under reduced pressure either can be used.

After the filtration, air is often taken in into the solution. Bubbles ascribable to this air give rise to the disorder of image in the inkjet recording in many cases and therefore, the above-described defoaming step is preferably provided separately. For the defoaming, a method of allowing the solution after filtration to stand may be used or various methods such as ultrasonic defoaming or reduced-pressure defoaming using a commercially available device can be utilized. In the case of defoaming by an ultrasonic wave, the defoaming operation is preferably performed for 30 seconds to 2 hours, more preferably on the order from 5 minutes to one hour.

This operation is preferably performed using a space such as clean room or clean bench so as to prevent mingling of dusts at the operation. In the present invention, this operation is preferably performed in a space having a cleanness degree of class 1,000 or less. The “cleanness degree” as used herein means a value measured by a dust counter.

In the present invention, the hitting volume of ink on a recording material is from 0.1 to 100 pl, preferably from 0.5 to 50 pl, more preferably from 2 to 50 pl.

The present invention is not limited on the inkjet recording system and is used for a known system, for example, an electric charge controlling system of jetting out the ink by using the electrostatic induction force, a drop-on-demand system (pressure pulse system) of using an oscillation pressure of a piezo element, an acoustic inkjet system of converting electric signals into acoustic beams, irradiating the beams on the ink and jetting out the ink using the radiation pressure, and a thermal inkjet (bubble jet) system of heating the ink to form bubbles and utilizing the generated pressure.

The inkjet recording system includes a system of ejecting a large number of small-volume ink droplets of so-called photo ink having a low concentration, a system of using a plurality of inks substantially the same in the color but different in the concentration and thereby improving the image quality, and a system using a colorless transparent ink. The hitting volume of ink is controlled mainly by the printer head.

For example, in the case of a thermal inkjet system, the hitting volume can be controlled by the structure of the printer head. That is, the ink can be hit in a desired size by changing the ink chamber, heating section and nozzle size. Also, even in the thermal inkjet system, the ink can be hit in a plurality of sizes by providing a plurality of printer heads differing in the heating section or nozzle size.

In the case of a drop-on-demand system using a piezo element, the hitting volume can be changed by the structure of printer head similarly to the thermal inkjet system, however, by controlling the waveform of driving signals for driving the piezo element, the ink can be hit in a plurality of sizes using printer heads having the same structure as described later.

In the present invention, the ejection frequency on hitting the ink on a recording material is 1 KHz or more.

In order to record a high-quality image like a photograph, the hitting density must be 600 dpi (number of dots per inch) or more so that an image having high sharpness can be reproduced by small ink droplets.

In hitting an ink by a head having a plurality of nozzles, the number of heads which can be driven at the same time is restricted, that is, from a few tens to about 200 in the case of a type where a recording paper and a head are moved in the directions orthogonal to each other, and a few hundreds even in the case of a type called line head where the head is fixed. This is because the driving electric power is limited or due to the effect of heat generated in the head on the image, a large number of head nozzles cannot be simultaneously driven.

By increasing the driving frequency, the recording speed can be increased.

The hitting frequency can be controlled, in the case of a thermal inkjet system, by controlling the frequency of head-driving signal for heating the head.

In the case of a piezo system, the hitting frequency can be controlled by controlling the frequency of signal for driving the piezo.

The driving of piezo head is described. The hitting size, hitting speed and hitting frequency are determined in a printer control section based on the signal of an image to be printed, and a signal for driving a printer head is prepared. The driving signal is supplied to the printer head. The hitting size, hitting speed and hitting frequency are controlled by the signal for driving the piezo. The hitting size and hitting speed are determined by the shape and amplitude of the driving waveform, and the frequency is determined by the cycle period of signal.

When the hitting frequency is set to 10 KHz, the head is driven every 100 micro-seconds and one-line recording is completed in 400 micro-seconds. When the travelling speed of the recording paper is set such that the recording paper moves {fraction (1/600)} inch, namely, about 42 micron per 400 micro-seconds, the printing can be attained at a speed of one sheet per 1.2 seconds.

With respect to the constitution of printing apparatus or printer to which the present invention is applied, the form disclosed, for example, in JP-A-11-170527 is suitably used. With respect to the ink cartridge, the form disclosed, for example, in JP-A-5-229133 is suitably used. With respect to the suction and the constitution of cap or the like covering the printing head 28 at the suction, those disclosed, for example, in JP-A-7-276671 are suitably used. In the vicinity of head, a filter for eliminating bubbles, disclosed in JP-A-9-277552, is suitably provided.

Also, the surface of nozzle is suitably subjected to a water repellent treatment described in Japanese Patent Application No. 2001-016738. The present invention may be used for a printer connected to a computer or for an apparatus specialized for the printing of a photograph.

In the inkjet recording method of the present invention, the ink composition for inkjet recording is preferably hit on a recording material at an average hitting speed of 2 m/sec or more, more preferably 5 m/sec or more.

The hitting speed is controlled by controlling the shape and amplitude of the waveform for driving the head.

Furthermore, by using a plurality of driving waveforms and selecting appropriate waveforms, a plurality of ink droplets differing in the size can be hit with the same printer head.

EXAMPLES

The present invention is described below by referring to Examples, however, the present invention is not limited thereto.

Example 1

A color ink solution was produced by the following formulation.

The conditions in the process of producing an ink solution were as follows. After adding and mixing all constituent components shown below, deionized water was added to make a total amount of 1 liter and the resulting ink solution was stirred for 1 hour under heating at 30 to 40° C. Thereafter, the ink solution was filtered under reduced pressure through a microfilter having an average pore size of 0.25 μm to prepare an ink.

TABLE 25 Formulation of Color Inks Light Light Dark Magenta Magenta Cyan Cyan Yellow Yellow LM-1 M-1 LC-1 C-1 Y-1 DY-1 Dye M-1 M-1 C-1 C-1 Y-1 Y-1 10.5 g 31.5 g 18.0 g 54.0 g 38.0 g 35.0 g C-1  2.0 g Urea 12.5 g 37.5 g 12.5 g 37.5 g 37.5 g 37.5 g Water  400 ml  400 ml  400 ml  400 ml  400 ml  400 ml At this step, dissolved (irradiation with ultrasonic wave for 15 minutes). DEG  130 g  130 g  100 g  100 g  100 g  100 g GR  150 g  150 g  150 g  150 g  140 g  140 g TGB  130 g  130 g  100 g  100 g  100 g  100 g 2PD — —   50 g   50 g   50 g   50 g TEA  7.0 g  7.0 g  7.0 g  7.0 g  7.0 g  7.0 g SW 10.0 g 10.0 g 10.0 g 10.0 g 10.0 g 10.0 g Water was added to a finish amount of 1,000 ml.

In the Table, the symbols indicate the following compounds.

-   DEG: Diethylene glycol -   GR: Glycerin -   TGB: Triethylene glycol monobutyl ether -   2PD: 2-Pyrrolidone -   TEA: Triethanolamine -   SW: SURFYNOL STG

The obtained inks each was filled in a cartridge for the corresponding color ink of an ink jet printer PM-950C manufactured by EPSON and a monochromatic step patch image of each color was printed. At this time, printing was also performed in the same manner using pure inks of PM-950C.

The image-receiving sheet used was an ink jet paper photo gloss paper “GASAI” produced by Fuji Photo Film Co., Ltd. When 24 hours were passed after printing of the step patch image, the reflection density in each density region of the step patch portion was measured using a densitometer X-rite 310 equipped with a Status A filter.

In this measurement, the portion where the density (Di) by the Status A filter light corresponding to the absorbance region of each patch was from 0.9 to 1.1 was selected. This sample was subjected to a discoloration test by storing it in an ozone gas discoloration tester capable of adjusting the ozone gas concentration always to 5 mg/liter. A commercially available ozonizer employing a high-pressure discharge system with application of a 5 kv a.c. voltage was used and the ozone gas concentration was set and controlled by an ozone gas monitor (Model OZG-EM-01) manufactured by APPLICS. After the storage, the density was measured in the same manner every each day to determine the density (Da) after storage and the time until the dye residual ratio Da/Di*100 became 80% was measured. Assuming that the relationship between density and time of the discoloration reaction is in accordance with a primary reaction, the discoloration rate constant k_(d) (h⁻¹) was determined from the time measured.

TABLE 26 Ink k_(d) (h⁻¹) LM-1 2.8 × 10⁻³ M-1 2.9 × 10⁻³ LC-1 2.5 × 10⁻³ C-1 2.4 × 10⁻³ Y-1 8.2 × 10⁻³ DY-1 9.5 × 10⁻³ Light magenta of PM-950C 2.0 × 10⁻² Magenta of PM-950C 3.5 × 10⁻² Light cyan of PM-950C 9.5 × 10⁻³ Cyan of PM-950C 1.0 × 10⁻² Yellow of PM-950C 4.2 × 10⁻² Dark yellow of PM-950C 5.7 × 10⁻²

It is seen from the results in the Table that the images formed from cyan or magenta ink (LM-1, M-1, LC-1, C-1) have high fastness as compared with Y-1 and DY-1 and the ratio to Y-1 having a maximum k_(d) value is 0.4 or less in all cases. On the other hand, the images formed from inks of PM-950C all are low in the fastness and have a large k_(d) value.

Therefore, LM-1, M-1, LC-1 and C-1 each was mixed with the corresponding ink of PM-950C at a ratio of 3:1 to prepare inks (shown below) balanced with the discoloration rate of Y-1, as the ink of the present invention.

TABLE 27 Newly Prepared Inks Ink Contents k_(d) (h⁻¹) LM-2 LM-1 + light magenta of PM-950C 7.9 × 10⁻³ M-2 M-1 + magenta of PM-950C 7.5 × 10⁻³ LC-2 LC-1 + light cyan of PM-950C 8.3 × 10⁻³ C-2 C-1 + cyan of PM-950C 8.0 × 10⁻³

As seen in this Table, the k_(d) value of each ink in the Table became from 0.8 to 1.2 times that of Y-1. Using these inks, an ink set shown below was produced and a portrait of a person was printed.

TABLE 28 Ink Set Contents IS-1 (Comparison) LM-1, M-1, LC-1, C-1, Y-1, DY-1 IS-2 (Comparison) Pure ink set of PM-950C by Epson IS-3 (Invention) LM-2, M-2, LC-2, C-2, Y-1, DY-1

The samples printed using these ink sets were subjected to a discoloration test by storing them in the above-described ozone gas tester adjusted to stationarily give an ozone gas concentration of 5 mg/liter. The images were evaluated after 3 days and 7 days of storage.

The rating was A when the portrait image observed with an eye after storage has good color balance, B when disruption of color balance was observed in the image, and C when the image quality was seriously deteriorated. The evaluation results are shown below.

TABLE 29 Before Start of Discoloration After 3 After 5 Ink Set Test Days Days IS-1 (Comparison) A B C IS-2 (Comparison) A C C IS-3 (Invention) A A A

It is seen from the results in the Table that when the ink set satisfying the requirement on the forced discoloration rate constant ratio specified in the present invention is used, an image having excellent color balance is maintained even after the forced discoloration test in an ozone gas atmosphere.

Example 2

Ink Set IS-4 using 6 inks of Y-1, DY-1, LM-3, M-3, LC-3 and C-3 was produced. The inks LM-3, M-3, LC-3 and C-3 each was prepared by mixing LM-1, M-1, LC-1 or C-1 with the corresponding ink of PM-950C to adjust the ratio of k_(d) to k_(d) of Y-1 to 0.5. The mixing ratio was determined by setting a rough ratio while referring to the relationship between the mixing ratio (3:1) of LM-2, M-2, LC-2 or C-2 and the k_(d) value in Example 1 and adjusting it based on an experiment. Using this ink set, a forced discoloration test with ozone gas was performed in the same manner as in Example 1. In the evaluation with an eye, rating was A after 3 days and B after 5 days. Even when the image was exposed to a high ozone gas concentration of 5 mg/liter for 5 days, the rating was B. From this, it is judged that the oxidation resistance of Ink Set IS-4 is in the practical level.

The image obtained by inkjet recording using the ink set of the present invention, which comprises at least three kinds of inks and in which the ratio of forced discoloration rate constants of any two inks is from 0.5 to 2.0, can maintain the image quality with less change in the color balance even when the obtained image is stored in an oxidative atmosphere.

The entire disclosure of each and every foreign patent application: Japanese Patent Application No. 2002-214066, from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth. 

1. An ink set for inkjet recording, comprising at least three kinds of inkjet inks, each of which includes a coloring agent dissolved or dispersed in an aqueous or oily medium and has a maximum absorption spectrum in a spectral absorption region different from each other, wherein, when a photographic printing is performed on a reflection-type image-receiving medium using the ink set and a forced discoloration rate constant with an ozone gas of each ink is determined in each printed region of said at least three kinds of inks, the ratio of any two of the forced discoloration rate constants is from 0.5 to 2.0.
 2. The ink set for inkjet recording as claimed in claim 1, which comprises at least one cyan ink, at least one magenta ink and at least one yellow ink.
 3. The ink set for inkjet recording as claimed in claim 1, which comprises at least one black ink.
 4. The ink set for inkjet recording as claimed in claim 1, which comprises two cyan inks, two magenta inks and two yellow inks.
 5. The ink set for inkjet recording as claimed in claim 1, wherein the coloring agent is a dye.
 6. The ink set for inkjet recording as claimed in claim 1, wherein the coloring agent is a pigment.
 7. The ink set for inkjet recording as claimed in claim 1, wherein the coloring agent includes a dye and a pigment.
 8. The ink set for inkjet recording as claimed in claim 1, wherein the ratio of any two of the forced discoloration rate constants is from 0.7 to 1.4.
 9. The ink set for inkjet recording as claimed in claim 1, wherein the ratio of any two of the forced discoloration rate constants is from 0.8 to 1.25.
 10. The ink set for inkjet recording as claimed in claim 1, wherein each of the at least three kinds of inkjet inks contains the coloring agent in an amount of 0.2 to 20 wt %.
 11. The ink set for inkjet recording as claimed in claim 1, wherein the at least three kinds of inkjet inks contains a compound represented by the following formula

wherein A represents a 5-membered heterocyclic group; B¹ and B² each represents a nitrogen atom, ═CR¹— or —CR²═, and when either one of B¹ and B² represents a nitrogen atom, the other represents ═CR¹— or —CR²═; R⁵ and R⁶ each represents a hydrogen atom, an aliphatic group, an aromatic group, a heterocyclic group, an acyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a carbamoyl group, an alkylsulfonyl group, an arylsulfonyl group or a sulfamoyl group, and the hydrogen atom of each substituent may be substituted; G, R¹ and R² each independently represents a hydrogen atom, a halogen atom, an aliphatic group, an aromatic group, a heterocyclic group, a cyano group, a carboxyl group, a carbamoyl group, an alkoxycarbonyl group, an aryloxycarbonyl group, a heterocyclic oxycarbonyl group, an acyl group, a hydroxy group, an alkoxy group, an aryloxy group, a heterocyclic oxy group, a silyloxy group, an acyloxy group, a carbamoyloxy group, an alkoxycarbonyloxy group, an aryloxycarbonyloxy group, an amino group, an acylamino group, a ureido group, a sulfamoylamino group, an alkoxycarbonylamino group, an aryloxycarbonylamino group, an alkylsulfonylamino group, an arylsulfonylamino group, a heterocyclic sulfonylamino group, a nitro group, an alkylthio group, an arylthio group, a heterocyclic thio group, an alkylsulfonyl group, an arylsulfonyl group, a heterocyclic sulfonyl group, an alkylsulfinyl group, an arylsulfinyl group, a heterocyclic sulfinyl group, a sulfamoyl group or a sulfo group, and the hydrogen atom of each substituent may be substituted; R¹ and R⁵, or R⁵ and R⁶ may combine to form a 5- or 6-membered ring.
 12. The ink set for inkjet recording as claimed in claim 1, wherein the at least three kinds of inkjet inks contains a compound represented by the following formula (I):

wherein X₁, X₂, X₃ and X₄ each represents —SO-Z, —SO₂-Z, —SO₂NR₁R₂, a sulfo group, —CONR₁R₂ or —CO₂R₁; Z represents an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group or a heterocyclic group; R₁ and R₂ each represents a hydrogen atom, an alkyl group, a cycloalkyl group, an alkenyl group, an aralkyl group, an aryl group or a heterocyclic group; when a plural number of Zs are present, the plurality of Zs may be the same or different; Y₁, Y₂, Y₃ and Y₄ each represents a monovalent substituent; when a plural number of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s are present, the plurality of X₁s, X₂s, X₃s, X₄s, Y₁s, Y₂s, Y₃s or Y₄s may be the same or different; M represents a hydrogen atom, a metal atom, or an oxide, hydroxide or halide thereof; a₁ to a₄ and b₁ to b₄ each represents the number of substituent X₁, X₂, X₃, X₄, Y₁, Y₂, Y₃ or Y₄, and a₁ to a₄ each represents an integer of 0 to 4 but all of a₁ to a₄ are not 0 at the same time; b₁ to b₄ each represents an integer of 0 to
 4. 13. An inkjet recording method, comprising forming an image using the ink set for inkjet recording claimed in claim
 1. 